Kid-proofing a PC

I recently bought Robin a laptop, which meant that her 3+ year-old PC is now freed up to do the thankless job of being the kids’ computer. It’s running WinXP home, and has a wifi connection to the home network, and before I let the kids run loose on the thing, I’d love to hear from any of you who’ve been through this process.

First, the stats: my oldest son is 6, his brother is 4 and Becca’s a couple years from being on a computer. The computer has plenty of RAM (half a gig), a decent processor (1+ ghz, if memory serves) and lots of hard drive space. Most of what the kids will be doing for the next year or so will be playing a handful of Windows-based games (educational, nothing too taxing on the graphics processor), visiting a few family-friendly websites, and, in Ricky’s case, possibly sending/receiving emails to family members. Basic Internet access for playing chess with me when I travel would be cool too.

I’m not particularly interested in locking this thing down tighter than a drum (that’ll just turn me into 24×7 tech support for yet another PC, something I really don’t want to have happen), but I am interested in taking basic precautions to keep them safe and in avoiding the inadvertent “oops, I deleted the hard drive” kind of stuff. So… what have you done?

Thanks for any suggestions.