defragment it...

well it depends on the type of computer.

delete unused files, temporary internet files, etc.

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http://onecare.live.com/site/en-gb/default.htm

it is made by Microsoft and is a free online scanner!

click full service scan as this will scan for Viruses, Spy-ware and will also clean up your computer from unwanted things like temporary Internet files which could clog up your computer and slow it down.

If you want to tune up your computer the full service scan also can do that.

You can also use Disc Defragment

Start | all programs | accessories | system tools | Disc Defragmenter

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Chicken

1. Scanning- Scan your computer for viruses, spyware, malware and the like. These are some of the main causes for slowing down a PC. You may try programs such as Ad-Aware, AVG anti spyware and other free programs like that. Update your anti-virus program and scan your computer or look for an online anti-virus scanner such as Trend Micro.

2. Delete old files - Remove  any programs that you no longer use, remove any files that you really do not want or just burn them to disks.

3. Disk Cleanup - You may use the Disk Cleanup Feature in My Computer by:

Clicking Start, point to All Programs, point to Accessories, point to System Tools, and then click Disk Cleanup. If several drives are available, you might be prompted to specify which drive you want to clean.

Alternatively you may install a program called CCleaner, which removes, in my opinion, far more stuff than disk cleaner ever will.

4. Defragment your Hard Drive - After a while of use a hard drive will become fragmented meaning the files are literally all over the place, which will slow down the access time when running programs. So to speed this up you can Run the computer Disk Defragmentation program which will bring those files together and decrease the access time for them.  

Click Start, point to All Programs, point to Accessories, point to System Tools, and then click Disk Defragmenter.

5. Correct Hard Disk Errors -  In addition to running Disk Cleanup and Disk Defragmenter you may also verify the integrity of the drive and fix any HDD errors you may have.

To run the Error Checking utility:

1.Close all open files.

2.Click Start, and then click My Computer.

3.In the My Computer window, right-click the hard disk you want to search for bad sectors, and then click Properties.

4.In the Properties dialog box, click the Tools tab.

5.Click the Check Now button.

6.In the Check Disk dialog box, select both of the options, and then click Start.

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