Do you know how much RAM your computer has installed? If you do not you can find out by going to Control Panel > Performance and Maintenance > System. On the general tab, towards the bottom you should see X.XGhz, XXX of RAM. I'm interested in the number directly before "MB of RAM."
After you confirm how much RAM you have in your system open the Task Manager by pressing Ctrl+Alt+Del, then choosing "Task Manager" if it is not already open. Click on the Processes tab, then click "Mem Usage" twice. Jot down the names (like firefox.exe) of processes that are using over 20,000 k of memory.
You should be able to set the page file size by going back to the System Properties box (Control Panel > Performance and Maintenance > System), choosing the Advanced Tab, then Settings under Performance, then the Advanced tab, then under Virtual Memory there is a change button that will allow you to set the size of the page file.
If you can post the amount of RAM, and processes someone will be able to help further.