How has it slowed down to a ridiculous level? Just on start-up or all the time?
My laptop did this recently – slowed to a veritable crawl, particularly on start-up.
Trying to work out why this had happened, I realised I had dumped a ‘temp’ photo folder on the desktop and hadn’t removed it. It was a large folder and once it was removed, normal service was resumed.
I can’t remember they whys and whatnots, but I remember reading ages ago that storing a lot of files on the desktop really slows the computer down.
There are conflicting views on the interweb about whether it should cause a problem, given that ‘the desktop’ is just another folder location really, but for my laptop it made a huge difference.