Jc,

Just a few things for you to try:

Before playing any games, restart the machine to gain the most resources, then shut down all background programs using Ctrl-Alt-Delete and "End Task" on all listed programs except for Explorer and Systray. If you're playing ONLINE games, you'll want to keep the antivirus program running.

Next, start by stopping the unnecessary start up programs from running by following the steps in the link below:

How To Shut Down Unnecessary Start Up Programs
http://reviews.cnet.com/5208-6121-0.html?forumID=45&threadID=22053&messageID=238474
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After that, make sure to clean out all the junk files on the computer, clean out the Temporary Internet Files folder, delete all .tmp files on the machine, then run Scandisk and Defrag to optimize the hard drive.
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Have you thought about increasing the RAM on that machine? Although 256 MB is good for general uses, when you're playing games, you might want to kick it up a bit.
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Have you installed the latest video drivers for that card?

http://www.sis.com/products/graphics/315.htm
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Do you have the most recent DirectX installed for your machine?

http://www.microsoft.com/windows/directx/default.aspx?url=/windows/directx/downloads/default.htm

Hope this helps.

Grif