Dedicate a partition or separate harddrive (D: is used in the following, but it can be any drive of course) with directories for
data (move your Documents and Settings folder there!)
portable (portable software)
images (system images)
When setting up a new system, use an imager software like acronis trueimage to make images after the clean install but with all drivers loaded, with the most important work software on it, etc. Update the image about every month.
When (not if) the pc gets slow
make another incremental image of your system partition (in case you had some data on your system drive that was not yet moved to the data drive.
make a current backup to an external drive and disconnect it!
copy the last smoothly running image onto your system disk. Take extra care and be _sure_ to have a current backup in a secure place!
have as few programs installed as possible and use portable applications whenever possible. Those can be installed to the D:\portable directory
If the system is slow in starting up or if you have disk activity which you cant explain, use the sysinternals autoruns program to find out and remove unwanted and unneeded programs. Best candidates are all those updaters that get installed alongside Acrobat, Java etc.