Virtual Memory essentially is a folder used by Windows in a manner similar to hardware, or RAM memory. It is very logical a machine running several browser windows, some background applications, and a download or two mught report "Low Virtual Memory". If your operating system s XP, and you use "Hast User Switching", that feature itself can eat up a buncha memory.
While usually best left for Windows to manage on its own, the user can set virtual memory size manually. In XP, Right-Click "My Computer", Select "Properties", open the "Advanced" tab, then, in the 1st panel, "Performance", click "settings" to pen the Performance Options Control Panel. Click the "Advanced" tab. In the bottom panel, "Virtual memory", click "Change". Make sure your root drive (Usually "C:
") is highlighted, click "Custom Size", and set the initial size to whatever you want - a common recommendation is twice your physical memory, the maximum size to twice that. Click "Set" >OK>"Apply">"OK" and reboot when prompted. That may or may not help - some experimenting might narrow down what works best for you.
Some folks feel once in a while, its a good idea to purge the page file, which is done by selecting "No paging file", rebooting into Safemode, resetting the page file either to your custom settings or the system default, whichever you use, and rebooting normally.
Personally, I use a large, fixed-size paging file - 4 times my installed physical memory. I purge/reset it from time to time. Dunno if it really does much good; its just a habit, but I don't have "Low Virtual Memory" problems, and it seems my practice causes no harm. I will say I have a big hard drive, with one partition allotted to Windows, applications, and the page file only, and lots of very fast physical memory.
Alex Nichol has a pretty good discussion of Virtual Memory HERE