significant hardware change
OR
something has gone boobies skyward and stability / reliability an issue
These are the only two reasons I would re-install my OS
My method
1. Install basic OS (no drivers / updates yet)
2. Install acronis and take first full image of system.
3. Install drivers , then take new incremental image.
4. Install / copy(a lot of progs dont use reg keys and can be copied from one install to another) in most programs I use daily , take another incremental image.
5. Install rest of programs I use , another incremental image taken.
Now if I install new apps I would make a new incremental image, OR
if I am about to try a new driver then I would inc backup first .... then play driver roullette.