128mb DDR will not cut it nowadays for todays programs, and the amount that is standard now is 512mb to 1Gb depending on how much programs you use.
Anyway though, changing the page file is rather simple.
Go to the Control Panel, System, and then when this window opens up, go to the Advanced tab, and click on Settings next to Performance.
On this Window, go to the Advanced tab and where it says Virtual Memory, click Change.
You can now increase the Virtual Memory to a desired amount. I wouldn't advise setting it too high, but just enough to make sure all programs can run withut the message popping up.
At the bottom, it should have a recommended figure which might help you decide.
I hope this helps