The key to this is "used much less frequently". As you use the systen more and more, the files become fragmented, and possibly corrupted.
1) Run Scandisk from [Programs]->[Accessories]->[System Tools], then run [Disk Defragmenter] from the same menu.
2) Also check for orphaned files in the "C:\windows\temp" folder (it should be empty when Windows is restarted.)
3) Unload uneccessary application at start-up, click [Start]->[Run], type "msconfig" and press [OK].
4) In [General], select [Selective StartUp] and uncheck [Process Config.sys] and [Process Autoexec.bat].
5) Uncheck [Load Startup group items], and click [OK].
The above few points is a guide for maintenance for the system software, by following the steps above should be able to speed up the system. It is also recommended to do some housekeeping on the software installed, i.e. remove/uninstall any unneccesary software so as to free up more disk space and speed up the system.