Definitevily (or whatever the spell is, I don't want to check it out now) it is a explorer.exe problem. Your explorer system is crashing and Xp restart it. Usually it won't give you a warning, and I can really tell you, I am an expert on it. I always make my explorer crash. The problem with me is usually that I over use my PC. I am using usually 210 MB RAM but I only have 128 MB install (I am waiting for $5 256MB DDR chips ). Also it could be a bad instalation of some program or XP itself. Back up all your important data and make system restore. If that doesn't fix, and you recall the problem begining earlier in the use of your PC, just reinstall XP. Actually I used to re-install Win98 a lot because of that. Kazaa had the ability to brake my whole windows instalation. Demos of games and programs can do the same, because they might be in beta (or alpha) stage, so they are unstable. Check how much resources is using your PC with the task manager. You might be using all your RAM, or your CPU is overhauling. When you say a K7-800, you mean A Duron or an Athlon? 800 should be enough for some task, but if you decided to have too many resident app. and on top of that you do a lot of multitasking, both, your RAM and CPU will over work.
There is also a big influence, which is usually overlook: Your power supply. If your power supply is not giving a healthy electricity to your motherboard, you will likely run into this problem. And believe it or not, this is a very common problem with OEM PCs, where they put cheap power supplys, into the systems. I have a Celeron 566 MHZ and I had a lot of trouble with it. I decided to build a PC and I begun with the case. I got one from Antec, and I used that unilt I got the Mobo and new CPU. During the month I used that Case, I hadn't a single problem. As soon as I put it back it, it will lock when playing some games, and my explorer.exe would crash sometimes (and I had 256 MB RAM). Now I changed the power supply, and I think my Celeron kick ass as much as my Athlon XP.
How to tell it is a power supply? Difficult. Mostly when you don't have a pattern for the problem, but it is always easier, if you have a good power supply that you can trade in (ask a friend to swap with you). Also check how much wattage are you consuming.!