If you go to your Start menu and type "Disk" into the 'Search programs and files' box, it should come up. (Disk Defragmenter should also show up - it wouldn't hurt to run it, but wait until after the Disk Cleanup.) Anything temporary is supposed to delete automatically, and if it hasn't, you should mark it for deletion. The Recycle Bin should be opened and given a quick look to make sure you want to delete everything in there, and then dump it. Thumbnails can be deleted - the ones that you need/use frequently will be re-created.
More than likely, those will account for the majority of the files that Disk Cleanup looks for, and could clear out a few megs of disk space. This can be used for various data operations, which can speed things up.
Running Disk Defragmenter will tell you if/how badly your disk is fragmented before it does anything. Defragmenting takes all the bits of each file and puts them in the adjoining physical areas of the disk - which speeds things up.
If you go to your Control Panels, select System and Security, and then System, it should give you information about the computer. Look for something that says 'Processor' and it will give you the processor's speed in mhz or Ghz. The RAM should also be listed on that screen to finish getting the info Hairy asked about.