My PC is canned regularly, why is it getting slower?

Someone asked me about why his PC is getting slower, even the PC is scanned daily.

Several reasons may cause the computer to slow down:

1. Too many software programs have been installed as starting at boot.
2. Hard disk drive is short on available disk space.
3. Data Corruption.
4. Computer is overheating.
5. Corrupt OS.
6. Bad Hardware.

The most common cause is #1. too many software programs have been installed as starting at boot. Once the program is installed as starting at boot, that program is having a piece of the precious resource, regardless of whether you use it or not. The evidence can be found in the increasing number of running tasks in Windows Task Manager (by ctrl-alt-delete). Over time, more and more programs were installed and started at boot, hence the PC gets slower and slower.

It is really difficult to find out which process (name) is what program is almost impossible. The easy way to regain those lost resources, i.e. to speed up your PC, is to NOT starting all applications at boot, or clean boot. In the mean time, these software are remian installed, but not started. They are evoked only when you need to.

Here are the related tech notes from Microsoft:
How to clean boot XP: http://support.microsoft.com/kb/316434
How to clean boot Vista: http://support.microsoft.com/kb/929135