My solution...
I have a MacBook 2.2 GHz with 4GB RAM running windows XP. Windows was very slow in general and when resuming from suspended state even slower still to recover (and sometimes never). This was a phenomena that occurred after my upgrade to Parallels 4 from 3, where I had no similar issues. I did two things that seems to have solved the problem foe me:
1. Under the control panel Systems -> Advanced Tab -> Performance Settings Button -> Performance Options Advanced Tab - Virtual Memory Button -> ensure "System Managed Size" was selected. For some reason "Custom size" had been selected (I do not remember ever setting this). In any event, under "Total paging file size" now indicates a range of 2 MB to 766MB with 767 MB currently allocated.
2. Under Parallels Desktop -> Preferences -> Memory -> selected Manually button and set to 1006MB. It had been set much lower MB-wise or Automatically before. Not sure if a somewhat lower manual setting than 1006MB won't work just as well.
3. For completeness here only (i.e., I made no changes to this setting, mostly because it does not appear that I can)... Under the Menu "Virtual Machine" -> Hardware-Memory reads Main Memory as being 512 MB. I do not understand the difference between this setting and the one under paragraph 2 above, but it made no difference since I cannot adjust it anyway.
Best of luck.
Jim M.