It won't matter as long as you use disk images (complete snapshots of your hard drive at a given time) and keep all large files on a separate partition (PC or Mac), so you can keep your disk images small and simple.
Disk images are crucial for any serious IT operation, and are cheap and easy enough to do at home. If you only have one computer and viruses or malware/spyware blow it up, just load your bootable DVD with the image file and software and you're back in business in 15 minutes like it never happened (as long as your working files are on a separate partition, very important!).
On the PC use Ghost or CasperXP (not very expensive), and on the Mac (yes, imaging on the Mac) use the free Carbon Copy Cloner.
Mac's, due to the UNIX layer and general re-design, are now just about as complicated as PC's have always been from an OS repair point-of-view. No more simple "clean install" options like in the good old days. If you don't believe me try messing with your current Mac's OS structure like you used to.
Anyone installing and uninstalling game demos know how messed-up a system can get. Defragging only does so much for your performance over time. A nice re-image does wonders for your machine (or mac hine as some of you with Sirius might call it).