Currently RAM is about $40 per GB. For XP you should have 1GB, for Vista 2GB.
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Disable Programs to Free Up Memory
Disable programs that can be disabled to improve computer performance using msconfig.
First: Uninstall any programs listed in "step 1"
http://www.techsupportforum.com/security-center/hijackthis-log-help/15968-updated-important-read-before-posting-log.html
Run PCPitstop "full tests" and "running programs" in the results it will designate programs that can be disabled to improve computer performance. Run MSconfig and uncheck any program "that can be disabled to improve computer performance"
http://www.pcpitstop.com/
How to use MSCONFIG
http://www.netsquirrel.com/msconfig/
Using MSCONFIG to reduce background programs at startup
http://alri.org/step2/usingmsconfig.htm
TASK LIST PROGRAMS A-Z
http://www.answersthatwork.com/Tasklist_pages/tasklist.htm
http://www.castlecops.com/StartupList.html
http://www.pcreview.co.uk/startup/
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Per
http://askbobrankin.com/low_virtual_memory.html
Low Virtual Memory
If you still see the dreaded "Your system is low on virtual memory" message then the first thing to do is make sure your computer is configured to allow Windows to automatically manage the virtual memory. In Windows XP, follow these steps:
# Click Start, then open the Control Panel.
# Click Performance and Maintenance, and then click System.
# Click the Advanced tab.
# Under Performance, click Settings.
# Click the Advanced tab.
# Under Virtual memory, click Change.
# Under Drive [Volume Label], click the drive that contains the paging file (virtual memory) settings that you want to change. In almost every case, this will be your C: drive.
# Click to select the "System managed size" option, then click Set.
# Click OK three times and restart your computer.
When you allow Windows to manage your virtual memory, it should make the optimum amount of virtual memory available, by automatically shrinking or enlarging the paging file as needed. If that doesn't solve the low virtual memory problem, there are a few more things you can try.