My record with MiniMe, which was formerly a machine I used to run eggdrop and bnetD as well as IRC and ICQ, running Win98SE, lasted 11 weeks, 4 days (81 days) before the power went out...
None of my XP machines have passed 2 months, though all of them have exceeded 6 weeks at one point. I usually find a reason to restart, or do something stupid like trip the power breaker.
My server, sad to say, although it used to be rock solid (Its uptime was approaching 200 days), now goes down more then your cash flow goes down on the vegas strip. Everything autorestarts of course, but it currently lasts about 3 days between crashes.
Currently my win98 box is beating the crap out of all my other boxes, regardless of OS.. and is approaching a third month. XP keeps giving me bluescreens while playing games using Glide (I have a voodoo 5 in that box), but I blame it on video card drivers.