Nothing terrible since the amount of space for say Temp files, one of the category that can be cleaned up when you run Disk Cleanup, is set to a certain number of MB or GB (and you can alter that).
I don't know what version of Windows you have or how it is set up so when you say you run Disk Cleanup from the Start Menu it's not something I'm used to seeing or using. When I do run it I open My Computer, select the drive I want to clean up (if there is more than one drive) and right mouse click on it and select Properties on the menu. There is a button for Disk Clean there and when you click on it it runs and produces a check list showing how much it can remove in the different categories so you can select or unselect what you want done. Then you OK it and it cleans up.
The Compress files is another thing altogether and is an option on that Properties tab I mentioned. Knowing Microsoft it is probably checked by default and one of the first things I do is uncheck it since I am not short of drive space and compressing and decompressing to use all take time.
What kind of computer are you talking about? A laptop from what you say about the fan? How large is the hard drive? How much free space is on it when you open that Properties tab?
What kind of applications to you have on the PC? Do they generate a lot of files? Or do you store a lifetime of photos? I can't imagine getting over a million files.
Hugh | 
