I usually install more than 150 programs on my XP,after which an annoying bug appears (on ALL my WinXP computers).
It seems Windows is only able to handle a certain amount of programs, and because I use my laptop these days for just about anything, I also install just about anything. Office packages, drivers,software tools, compression tools, often more programs that can do the same thing (like Ms Office VS OpenOffice) just to see the differences.
After installing a certain amount of programs (which is near to 150-200), usually some programs,drivers or software things just won't work properly anymore (like eg having issues with wireless connection or so).
What's even worse, I just found out that my HD's (200-250GB) that have data unused on them, get corrupted data over time, which makes me believe that a 6 months, to 1,5years period is really close to the maximum you should go without reinstalling everything.
Large HD's are always nice to have, but they are also much more prone to data loss over time.
Just to give you an idea, I left my HD's in a dark environment, ranging between 18 degrees and 27 degrees celcius.
I also tested drives in Florida leaving them in a closet space with temperatures between 72 and 89 degrees. It seems most of my harddrive start getting corrupted data after 1 year (Western Digital,Seagate, Maxtor); and start having unreadable sectors after a good year and a half.
These are serious reasons why many should reinstall every year or two.
Windows Vista is less because it uses constant defragging, so in essence before data goes bad it has been read and rewritten already.
One reason more to start thinking of storing information on flash drives,and start installing an OS on flash drives.