I wipe about every four months: late December, early May, and mid-August (corresponding to the end of fall semester, end of spring semester, and end of summer break, respectively). I've found that the list of 'core necessary programs' is far smaller than I initially anticipated. Usually my reinstalls look like this:
- OS (Windows Server 2008 R2 with all the cruft switched off)
- Internet (firefox + adblock + noscript) [first download is always firefox. No exceptions]
- Utilities (foxit reader for PDFs, some rar extractor, etc)
- LaTeX maker (TeXnicCenter + MikTeX)
- Data analysis software (Stata 11 + stat/transfer for weird file formats)
- Text editor (notepad++ or crimson editor)
- Sync utility (Microsoft SyncToy)
- Music player (foobar2000, though I'm looking around for an alternative)
- Movie player (Media Player Classic + CCCP. Handles every format with grace. VLC is the devil)
- Office suite (MS Office 2002)
Which totals about 10-12 downloads. The whole process takes less than an afternoon. I've even gone for a few months without installing Office, relying on LaTeX for documents and Stata for spreadsheets.
All of my core files are backed up onto a 4GB flash drive that is synced to my computer; I run sync checks pretty much every time I use the flash drive, so it's never more than a few hours out of date. Stable files (music, etc) are all backed up to an external hard drive.
I have a standing policy not to let a Windows install survive for more than 6 months. Things tend to randomly break or not work.