compressed folders n comp. crashing!

I downloaded a program called HijackThis to sort out some
problems, but i accidently right clicked on it, went to properties, and
changed the file type to a word document. The file was initially a
compressed folder, but when I clicked on change, it changed all the
compressed files on the PC to word files. I Don't know how to change the
files back to compressed files. When i right-click and go to new-
compressed folder, that also appears as a word file. How can I change this
back?
Also I have been experiencing problems with the computer crashing
continuously, over the last month especially. The mouse arrow goes crazy
over the screen, and then it just freezes, so i cant even close down any
programs or anything. Could this be related to the compressed files
problem? I thought it was probably a problem with the mouse at first,
but I am not sure.

Oh and thirdly, ive tried to go to system restore but it doesnt seem to
work, it says there are no dates in there even though i created a restore
point only a few months ago. It wont even let me create a new restore point.
Whats wrong with my pc!!??

Thank in advance, for your help!
Nish