I recently bought a new memory stick for my windows xp home. I have a 1.9 gHz processor and two hds both 80 GBs. When I purchased my computer it came with two 256 MB PC2100 RAM chips, and I went to my local best buy and asked a tech guy there what would be best for an upgrade. I ended up buying a new 512 MB PC2700 (which says it would be compatible with 2100). So I now have one 256 and one 512. When I'm playing a game it randomly freezes and I have to press the power button on my computer. (the whole screen is frozen, mouse won't even move). I've also noticed it seems to take a little bit longer to start up some programs now, and recently when I started windows just now, I logged onto my account and the explorer bar was completely dark blue, and wouldn't show up. I could access the taskmgr and I managed to log off then on again, and it showed up fine. Any ideas to what could be causing this or how to fix it? I assume it could be bad memory, but I really don't feel like trying to return it or wasting more of my money.

I can't tell what you have, might you know what kind of Ram you have? Like SD-Ram or Ram-bus? And could you tell me also what kind of graphics adapter you have? Perhaps then I can tell you more.

I dont think mixing the ram is a good thing, even if it says it is compatible.
did you try with just the 512 stick?