Well, your system should not be responding that badly. A couple things to check:
1) How much available hard drive space do you have. Even with a 500 GB hard drive, you really don't want to drop below 100GB (20%) of available space.
2) Make sure you still have all 4GB of RAM. One of your RAM sticks may have died.
Either of these scenarios could result in what you're experiencing. If your system hasn't been cleaned in awhile, you may want to consider doing that as well.