Hi Jonathan,
as I said, the 100% CPU explorer thing has many faces

The one I mentioned may be the 'easiest' to target, but there are more. From time to time I run into the spinning colored beachball situation too. I then go to the actvity monitor, let it show ALL processes (not just mine) and have look which one's the bad guy drawing so much power from my machine that other processes (even a simple file save) won't work or take ages.
You already did this and found the Finder process. The problem is that the Finder is a deeply system-integrated program responsible for much more than we can imagine. That means if there is something else going on deeply in the underground of the OS, people like you and I will unlikely find the 'real' reason for the Finder process amoking.
For example I never had an iDisk, but since this is a cloud space connected to your machine and showing up as something like a network drive, it's absolutely possible that the iDisk causes some pain to the Finder because there are constant delays while the Finder tries to gather the information it wants to display. Must not but can be.
At such a point and given this was my machine and after all I experienced up to now: My way is to save all my data to an external hard drive, insert the OS DVD an start over. Maybe it's a good idea to delete the whole system drive via the HDD utility software during the setup (where the menu bar comes up), and after your OS is set up and all updates through the software update utility are made, you have a fresh, clean system, probably without any problems at all.
At this stage, you can make a full (!) time machine backup as a blueprint for future use if something goes wrong.
The main idea now is to carefully, step by step, set up your desktop, your workspace as you are used to before. After every change, every step, control what happens. That's the way you can find your troublemakers. iDisk is one of those steps. If you ended your personal set up of the OS and still everything is allright, do a second full time machine backup to have a blueprint of this stage.
Then install your software, step by step and control every time if problems arise. If you're lucky you will experience no problems at all because maybe your last installation simply was messed up by trial and error.
But if problems arise, you can see with this method where they come from. This is what I mean with 'encircle'.
Given you were able to find and solve possible problems (or didn't see any at all), the next stage would be to set up a fresh VM with PD6. If you have the choice, try Win 7. XP is reliable but outdated, and Win 7 is the better OS when it comes to virtualisation.
My opinion - we have XP VMs in our company, and they work well, but since Win 7 is the 'modern' OS, all virtualisation software solutions have their emphasis there. Example is the PD6 SATA drivers for the virtual HDD in MS Vista and MS 7 VMs where all you can get in XP is normal IDE. I played around a little bit and found a way to bring SATA to XP VMs and can confirm it definetely was worth the long night it took to find out
Once that VM is finished, start it (Parallels tool normally automatically install) and do a little testing. Is it fast now (MUCH faster than before)? The answer should be 'yes' (if 'no' you need more professional help

).
Now that you know a smooth VM is possible, copy back your 'old' VM you already have and give that a try too. If it's too much faster than before - very good! If not, you can try to change the VM preferences (e.g. the performance tab) and see if it helps. If not, well, then throw it away and start with the fresh created VM, install the software you want to use inside etc.
By the way, if it's really Leopard (10.5) you have on your machines and not Snow Leopard (10.6) I would consider to buy the upgrade. In Germany it's about 30 Euros, small money, and 10.6. is said to be greately advanced over 10.5 in many meanings, virtualisation being one of them. I'm not quite sure about your situation because you mentioned your machines were quite new - Snow Leopard is distributed on all new Apple machines since End 2009. if I misunderstood, don't bother
A last suggestion concerning Bootcamp - as I mentioned before, if you don't have any special needs like hardware intensive 3D gaming or some special modelling software (Autocad, 3Dsmax etc) that need every little bit of raw power your machines can give, there is (IMHO) no need at all for a Bootcamp partition. As you will see, all 2D software like MS office is perfectly running in a VM near native speed. So if you don't need it, throw it away, and it will save you much time and effort
In my opinion Bootcamp is a very sexy toy Apple developed to make the decision towards a Mac easier. It simply means: 'You can do almost anything you want in the Mac OS, if you need Win software, there's virtualisation, and if you need the ultimate power, well, here's Bootcamp'. But to be honest - is it really charming to leave the Mac OS and restart every time you want Windows?
By the way, if you like to learn something about your Mac OS and have no 24/7 support like I luckily had when I started, there's a very comprehensive book series targeting the user who has the need to know something but doesen't want to read a 500 pages manual. The series is named '... for dummies'. Despite this slightly disrespectful title, these books are made very good. The series has many titles. The one fitting here would be
this one. Or the one about Snow Leopard if it's that you have. In my company I recommend these books for the people who need to learn something about their Win OS or MS Office and people are lucky with them.
So far for today - now that you know I'm from Germany you understand my long answering times ... it's simply the different time zones.
Best regards
Chris.
Maybe I can give you a recommendation when you give me some input what your primary software is in the Windows world.