You probably have too many programs running in the background, which is just too much for 256mb ram, but to access virtual memory..
Right Hand Mouse click "My Computer" then select "Advanced". Select Performance Settings. Then Advanced.
You should see Virtual Memory settings and how windows is set up to use memory on your pc. My advice to you is to select System Managed Size, then go back to the visual effects tab and select "Let windows choose whats best for my computer"