Could be anything. If the hard drive is noisy, then it's probably that on the way out. Your registry could be bulging big time. You could be running out of disk space. Swap file is too small. Not enough RAM (or a DIMM has failed, halving your memory...).
Take your pick.
With more details. If you're not sure, phone your friendly local PC man (not PC World). Sometimes these problems need someone on site to sort them out.
Or you could just reinstall.
Or maybe something has got into the startup folder which is too much for the pc to handle (prehaps badly written software downloaded fron the net).
In any case I would try starting in safe mode as this will only load the impotant stuff on startup. Then go into the msconfig and uncheck as much as you can.