Hi Tom. Ths may be no more than a fan running dry, but you're right to be concerned as, depending on the age of your machine, a hard drive failure could be on the cards. Any other symptoms - like error messages, system crashes, etc? 4 years is not that old for a computer (although it's a lifetime in the development of the technology), so you may be OK - see below.
First step is to protect your existing data - do you have a CD-R that you can write your files to? Programs don't matter as you have the original installation disks to reinstall from should the worst happen.
Having protected your data, might I (respectfully) suggest that you have a word with a local computer shop or group - they can diagnose for you and replace any defective components and reinstall from your old drive if necessary.