Jonny........................what OS?.............I suspect this is not your usual machine?

What are the physical specifications?

How much free hard drive space have you got?

Have you tried safe mode, defrag, then have a look at what is in your startup file?

Have you run a checkdisk on the HDD?

Also get: http://www.winpatrol.com

It is a sort of security tool but I find that I use it more for diagnostics and fault finding these days

Then you have a rat hunt a.k.a. as "clean out the crapware"

Give that lot a go and let us know how you get on.

Cheers

EDIT: SAFE MODE is your friend...................