Dust, is there any dust ...anywhere?
HDD free space? You need a minimum of 25%. Considering a 1 TB hdd, that would amount to 250gb of free space required (seems an awful lot of free space required, maybe Microsoft had not considered that there would ever be 1TB hdd's, and that we'd all be using 320 gb hdd's for ever).
CPU heat? You might need to re-apply thermal paste, some CPU's run hot and cheap or low grade thermal paste does dry out.