parallels desktop

i have parallels desktop on my macbook, and it has started to say that the virtual memory is low and that i need to change it.
i run the parallel in coherence with my mac, so i my dock to the left of the screen, and start menu down the bottom. when this message comes up, the start menu disappears, as well as any windows programs.
any way to fix this?

Hard to tell what's up from your brief description...

What Mac hardware are you running and what are it's spec's - specifically RAM and HDD rpm speed?
What applications are open in OS X while Parallels is running windows?
What hardware profile do you have your Windows installation on Parallels set to - specifically RAM allocated, performance options, etc?
What applications are you running in Windows at the time this happens?
What version of Windows are you running and do you have any software that auto runs upon start up (sucking up systems resources)?

I use a macbook (bought about may this year). it has 4GB of Ram and processor speed is 2.4GHz (is that the HDD rpm speed?).
i usually have both mac and windows running in coherence. i usually always have entourage, firefox, and itunes open. sometimes more sometimes less. but the error usually comes up when i have been away from the computer for a while..
the only thing i run in windows is live messenger.. i'm using windows xp, with nothing that auto runs on start up.
hope this helps