Hello, JT.
It sounds like you need to have the PC looked at. If the fans are running fast, that would indicate a heat problem. The heat problem can be due to dust build-up on the vents or an overactive processor. The processor may be overactive because the system is being worked hard by conflicting programs. This can also cause lockups and crashes. If your problem was just random lockups and it started after adding the memory, I'd suspect a bad stick of RAM, but the racing fan seems to rule that out. Note that adding memory would certainly help with system resources for all programs and I would not have pinned the system resource issue on OneCare as most people report that OneCare uses less resources than many competing protection products.
The link I provided is for the public newsgroups listing. I don't know that I'd recommend that you go there. You can try OneCare support, but I think the best place to start would be a local computer shop to have the hardware checked out.
-steve