I rarely have to use Windows at all - probably once every couple of months - so I've found it useful to keep a copy of VPC with a 500 meg image of win98 on a CD, already loaded up with all the utils I'm likely to use.
That way, it isn't using up space on my computer when I don't want it, but if I need it, I can just slip the CD in and run Windows straight off that! Handy, yah?
Incidentally, the publisher is Connectix.
Another option is Bochs, which can run windows but will require a lot more fiddling around. The advantage is that it is open-source freeware. The disadvantage is you'll still need to get your hands on a legit copy of windows and find a way to get it running, and that you won't be able to interface as neatly into your Mac system as you can with VPC. Bochs is an option, though, if money is tight.
And as for Windows XP, forget it! I've never seen an OS that sneers at the users quite the way XP does. Win 2000 is pretty good, and 98 or Me are handy for games, though not as stable.