depending on useage 4 or 6 times a year should be enough
if it's running slow
try
http://www.ccleaner.com/download/builds
just use the defaults ... see how it feels ...
if no joy then restore/reinstall
personally I split my drives into a couple of partitions ...
c: is system + basic software (stuff that changes infrequently)
d: is my working Data (I always move my documents here)
e: is games / tryout software and photo/music storage ready for archiving to CD/DVD (if required)