I think its a Ram issues too....
When you start up your machine, does the systems recognized the 600 meg of Ram?  Also, does the startup say you have a "1800+ AMD".  As you may want to check your BIOS to make sure the mhz & multiplers are set correctly.
Another thing, Compaq mainly uses "cheap" parts to build their systems.  As they don't usually put a high end motherboard in their systems.  So you may want to get another video card.  As most "All in one boards" have a 16 or 32 meg onboard video card and usually aren't the best for gaming.  3D or non-3D.
Also, I wouldn't be surprised in there's a 5400rpm Hard drive in there as well...  So check your BIOS setting and/or try another video card.
Hope this helps.
-GGKoul