If you can get it to boot go into uninstall programs and uninstall as much crap as you can ... thus freeing up space on the drive for it to cache the OS properly and run.
Then run cleandisk and defrag,
Then run your AV program and antimalware.
You might have a bit of malware or a virus that is chewing up cpu cycles and slowing it to a grinding halt.
Use hyjackthis to remove unwanted programs that have loaded at bootup.
Get some more RAM ... if you have less than 4GB.
Make sure you do not fill up the HDD to the point where it cannot cache the OS properly if you have less than 4GB of RAM.