It sounds like you're system is in a bit of a death spiral.. I would 1st and foremost be totally sure you have all your data backed up because at this point, one of these times it just may not start up for you.
Generally, because of how poorly windows cleans up after itself due to both windows and bad application deinstall routines, if you use your system heavily, especially if you install/deinstall programs somewhat often, it's just a good idea to re-core your system every couple of years, or more frequently.
As was posted earlier, if you're only running 2GB ram, your system is probably caching a good bit, and with the disk so full, if you don't regularly defrag, the caching will really bog down performance. To run windows with any decent performance, I never let the boot/caching disk get below 25GB free.
If you have not defragged, I would certainly try that, it alone may help the speed due to the above.