The box I use all the time (WinXP Pro SP2) and make changes to on a regular basis, gets fucked up after a while. I feel lucky to get a year out of it without a problem. I made many hardware changes to this thing and actually got an extra nine months or so without anything major going wrong. I was beside myself with glee for a few months. It's pretty stable for a five year old computer.
Then, out of the blue, weeks after anything changed ..... PROBLEMS!
My more modernish box, which is still four years old running Win XP Pro (but a cracked version) SP2, is loaded like a fatboy with graphics software (Photoshop, Illustrator, Corel Draw, Macromedia, Painter, Quark, a printer rip, about seven fractal generators, but NO NETWORK BS! In fact, I have ALL Windows services turned off that I can possibly get by without!) and I never have made any hardware changes to it since I put it together. It still works flawlessly after four years on its oringinal install.
The box I use for the kids educational games, most of which are Quicktime based and need Win9X to work, I re-install about every six months. Actually, I have fucked that one up myself on re-install by putting the games in - in the WRONG ORDER, FFS! That one suffers lots of conflicting changes to the software, though. Plus, the kids often destroy it by becoming impatient. The only time I have ever seen Windows warn me that my mouse buffer was over-run was after the kids had locked up the system.