Question: Is there some way to solve my problems without doing reinstallation of the OS?

There has been a progressive problem with my Win 7 desktop machine going more slowly and in turning off by itself. The turn offs may be after several hours but in the past few days often in less than an hour. Yesterday it turned off after 20 minutes. Today, it is running longer, happily.
The shut downs have been associated with the whirring of what sounds like a fan. It has sounds of slow, medium and breakneck speeds. Sometimes the sound will not start until the computer has been on for some hours; other times it starts with even the breakneck speed. My computer tech, who observed the sounds but did not have a solution (before the shutdowns started) described the sound as a flying saucer taking off.
I've observed that sound tends to increase when playing audio CDs, more so when connecting to the Internet (WiFi) and particularly when streaming video/audio via the Internet. It will also occur with less demanding work, such as with MS Word.
Other items I've noticed over the past year or so: Disappearance of program(s); renaming of a file: and moving a picture (in Picasa) to the place and name of another photo (which is no longer to be found).
I've run a good number of anti-malware programs, various anti virus scans, defrag and if there was anything else in Windows 7 maintenance, I did that too (except System Restore). Windows Firewall and Avast anti-virus have been in place for years. I've done all the backup work to prepare to reinstall the system but would like to avoid that chore if possible.
The Machine: Acer Aspire M1100 (desktop) running Windows 7 Home Premium (installed from OEM/retail disc over the pre-installed Windows XP) with Service Pack x. 6 GB RAM (my tech added 4 GB while mentioning that only 2 GB of it would be usable; the flying saucer sounds preceded this installation).
The HD is partitioned into C: and D: drives with the OS and some files on C:
Thanks for any help you can give.