Seems like it may be a hard drive issue.
Few things you can do to try and help this is: Go through your system, cleaning up any unnecessary programs and files that no longer need to be on there, or that can be burned to disc and stored.
Possibly use a program such as Ccleaner to get rid of temp files/old cookies and such that can take up a LOT of space (DO NOT USE THE REGISTRY FIXER/CLEANER for this program though as it could have conciquences!) Do note that Ccleaner will delete a lot of old temp files/cookies and a few other things such as History that Web Browsers use, so only use this program at your own risk.
Next you can do a Disk Defragment. This will take serveral hours to complete, specially if the system has been running for years without a reinstall or defragment before. This process will minorly help speed up the hard drive preformance.
The issue might also be that the hard drive is getting old and warn out, the discs are spinning fast and the reader head is moving to read the data, you may want to think about soon replaceing the hard drive, there are ways to "image" the hard drive over so you can keep your windows installation as well as all the data and programs on it. However if you are not familiar with doing this I would suggest having someone who works in the IT Field and is comfortable doing hard drive imaging do it..
It really does seem like a hard drive problem as when a program starts up and gets the computer going you hear a whirring (the hard drive discs spinning) and this happens with the age of the computer as well as if the drive is close to being full. Just some things to think about.