It randomly forgets the dimensions and position of my app windows. Occasionally at random times, but always when I reboot. And, c'mon, this is Windows. I have to reboot every time I look at my machine the wrong way.
On the quest to make XP a slightly smaller gigantic piece of ass, I'm using the BagMRU and Bags registry hack. It's working well, so far. *gasp* My windows are always the same size as when I closed them... the way they're supposed to be in the first place =O How. About. That.
There are a hillion-jillion other annoyances with XP, Office XP and the whole lot, but you just take your small triumphs, y'know?