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Thanks, everyone, for your suggestions. Yes, factor30, I run disk cleanup regularly and check the recycle bin - empty at the moment. I've transferred all my old photos and videos to a separate bit on the G drive, just keeping ones from this year on C in case I want to edit them. Hardly any itunes, so that wouldn't make much difference. The last time I ran ccleaner it freed up loads of space, but as I mentioned it also took away some things that I had to re-install.
The reason I'm thinking there isn't much room on C is because when I attempt to de-frag it tells me that to do it properly it needs at least 15% of space, and I'm well below that.
I'll go for the upgrade to IE8 after the weekend.