Desktop for Mac has slowed whole iMac down

Hello experts,

I was hoping someone might be able to help or offer some advice.

I've got an iMac 27", Intel Core i3 3.2GHz, 4GB DDR3 RAM, running Mac OS X Lion (10.7.5).
I've recently found the need to run a windows setup to run Sage Instant Accounts and also a piece of property management software, both of which only support Windows OS.

I've purchased Windows 8 together with Parallels Desktop v8 and installed the 64Bit version of windows on my one and only virtual machine, courtesy of Parallels.

Since having done so yesterday, my machine grinds to an almost halt when doing fairly simple tasks in OS X (such as opening new Google Chrome tabs, checking Mac Mal, changing tunes on Spotify, editing some stuff in Dreamweaver, etc).

I've given-up on trying the Coherence function - which would be of most use to me - as the machine just simply doesn't seem to have the available resources to cope with this.

Does anyone have any advice? My Mac ran absolutely fine before I installed Parallels and put Windows on it and, interestingly, runs absolutely fine when I "suspend" the virtual machine and just have Parallels running in the background and showing me the option to "Resume" my Windows 8 VM.

Any help gratefully appreciated!
Thanks in advance, folks.

Alex