I have been servicing computers for over 25 years. Hardware and software has changed considerably but people have not. They will always loose their software disks or forget what is important until it is gone. I had one of my technicitions reformat a customer's hard drive. He backed up what he thought was important (My Documents, Outlook PST file, Favorites, Desktop, QuickBooks file, etc..) but forgot to back up 20GB's of music files the guy had downloaded. Even though the music files had nothing to do with business (his company management probably would agree) he was pissed. We lost the customer because of this one event.
I have usually been able to clean a system to restore it to at or very near fresh-install speeds. Clean spyware, remove redundant scanning software (I found a system with three anti-virus programs once), defrag the hard drive and optimize the registry. A quick way to defrag is to Ghost over to a new drive. Windows is just a collection of program files and overlay libraries and a registry database. Keep track of what processes should be running and it's easy to find and remove what is slowing down you system.
Best idea: If you feel you *must* start over with a clean install, replace your hard drive. Hard drives are cheap and you can upgrade your capacity at the same time. This way you can keep your old hard drive as a backup and install it as a slave if you need to recover files, or re-install it as a master if you need to run an application you may have forgotten about.
Dial-up networking and POP accounts & passwords: Most people don't have a clue if you ask them and they don't document them. You can back up the account information in Outlook Express (Export account information in account settings). You can recover Dial-up Networking and Outlook passwords with freeware utilities. Document everything.
Some software has registration codes based on when you install (date & time & hardware configuration). Be prepared to re-register (and pay) for new registration codes.
Some software keeps its data files in with its program files (Like QuickBooks). Search the entire hard drive for needed data files.
This one tip will help you sleep better tonight
A few seconds are all you need to get a better night's rest.