Your actual memory (system memory or RAM) is 512MB. Whenever your computer needs to use more than 512MB of system memory, Windows XP uses some free space on your hard drive to act like extra system memory--that's Virtual Memory.
When your hard drive gets full, there is little free space for Windows XP to use as virtual memory, so you get a low virtual memory warning.
Because you stated that your paging file size changes, that tells me that that Windows controls the paging file size, which is good. If you set a paging file size yourself, Microsoft recommends that it be 1.5 times the size of your system memory.
BOTTOM LINE ADVICE: Set virtual memory to ''System Managed Size.'' Check your hard drive properties; if it is getting full, you need to get rid of a bunch of stuff or get a larger hard drive. This will improve system performance, in addition to getting rid of your virtual memory problem.
I scan for viruses/spyware daily on my older (2003) XP Emachines (1.79mgz processor/512k ram).
Whenever I've run a couple of scans the "low virtual memory" bubble pops up. A reboot takes care of the problem.
I don't really understand a lot about virtual memory and page files. I've tried reading up on this on the net, have located how to increase VM, etc. But, don't really (a) understand what it's all about or (b) whether it's even necessarily a problem in itself.
My PC's page-file usage is typically 275 when I call up task manager. Have no idea if that's a good or a bad thing. (Also, if this is helpful, my HD is 40gigs and my page file settings are 192 and 363 when I open properties).
I realize I may not even be giving the necessary information for help here, so please bear with me.