XP Game Crashes

I built my computer in November with these specs:

-Antec SOHO Server Case (4 5" drive bays, 6 3.5" drive bays, 4 fans [2 front, 2 back], 400W Power Supply)
-Pentium 4 2.66 Ghz processor (NOT OVERCLOCKED YET [past 3 Ghz], heatsink and fan included)
-MSI Neo2 865PE Motherboard (10 USB ports, Mouse, Keyboard, etc.)
-512 MB PNY DDR SDRAM
-ATI Radeon 9600 128 MB Gfx AGP Card
-Maxtor 80GB 7200 RPM HDD
-Liteon CD-R/CD-RW/DVD-R/DVD+-RW/DVD--RW Drive
-Generic DVD-ROM drive (can't remember the brand name)
-10 year old 3.5" Floppy Drive that doesn't work
-Windows XP Home

Sorry for all of the useless info, its just a copy/paste job from my own forums. (WHICH YOU SHOULD JOIN!)

I am having trouble with a few games, namely being MechWarrior 3, Zoombinis Mountain Rescue, Croc2, and LEGO Racer 2. When I start them, and I click EVEN ONCE, they crash out to the desktop with no warnings or anything. Its just a quick flash from whatever screen I was on, to a black screen, and then to the desktop. I'm not sure what's causing this and that's why I'm here. Hopefully you guys can help, because I'm clueless...