Disk Cleanup does the same sort of job as CCleaner. But CCleaner is more comprehensive.

To answer brianhutch's question, on the System tray, I have nine icons.

Maybe obscure, but I did see a machine that went from relatively speedy to dead-slow, simply because the son downloaded a game installer and saved it to the desktop. Once we moved that 6+ GB file to another folder, it was back to being speedy again.

Nine isn't dreadful - but it would be worth ensuring that they are all actually needed.

I would recommend the following:
- take a backup
- uninstall old application and games
- run CCleaner
- defrag

Note that it is important to run the CCleaner after uninstalling software - it'll clean up some of the mess left behind.

I personally hate things saved directly on the desktop. Shortcuts to things if you must, but not actual files. But that is just me.

Some games taking longer to start than others doesn't worry me too much, the same game suddenly slowing down on startup would be worrying - especially if it wasn't solved by a defrag. However - I have come across a few games take longer to load the more save games you have.

Instability is more worrying depending on what you mean by it.
- is it randomly crashing whilst running a desktop application (worrying)
- is it some game that crashes randomly (not so worrying)

Does the machine have good ventilation - sitting a laptop on a towel is not a good idea!.

Yes. I has a good space behind the fan.