Hmm... Well, there's nothing about your computer that really jumps out at me as being the culprit, but I have a few ideas.
1gb of RAM has become the standard for PC games; some of the really intense games have started to recommend 2gb. You only have 512mb. Civilization is a much simpler game in terms of required computing power than the other two you are trying to play. I would check the boxes that the games came in to make sure they don't require at least 1gb RAM to play.
If they do - we've found the problem. Here's what I would recommend:
Start a new post in the appropriate forum and ask if any of the hardware junkies could help you find the best RAM your motherboard can handle. Be sure to post the full name of your motherboard (like you did in this thread), let them know what RAM you already have in there (once again, like you did here), and make it clear that you need exactly 1gig.
If it turns out your MoBo supports a faster RAM (you have PC-2100, standard is PC-3200), you will be advised to buy two sticks of 512mb RAM and throw away what's currently in there. This is because RAM will only run as fast as the slowest stick, and twin sticks run much faster than one.
(I'm still new to these forums, so I don't know if there's a good place to start a thread like that @ CH.com, so
mods, please don't think I'm using your forums to advertise and delete this post. If you can't find a good place to post the question, or you aren't getting good answers quickly, goto
www.motherboards.org/forums and post in their "Recommendations" forum. There are about 12 people who stalk that area all day & *live* to help people build PC's)
If the other two games do not require a gig of RAM, then I don't know what's causing it. All I can recommend at that point is doing a little spring cleaning on your PC. Run a deep virus scan. Run Ad-ware removers. Run Spy-ware removers. (make sure you update all the virus/adware/spyware programs you have first)
Go through every piece of hardware on your computer and make sure you've got the latest drivers for *everything* (video, audio, internet, motherboard, cd drive, keyboard, mouse, joysticks, monitor, etc.)
Go through your add/remove programs list and uninstall everything you aren't using. Look through your personal files and delete everything not worth holding onto (old pics, movies, music, etc). After all that, defragment your hard drive.
All that will probably take you a good 2 or 3 days to complete, but when your done you should notice dramatic impovements with performance. That extra "edge" could be all your computer needed to get the game to run properly.
Let us know what you find, and good luck!