My computer is performing slowly, how can I fix this?

Method 1 : Disk Cleanup
1) Click Start > All Programs > Accessories > System Tools > Disk Cleanup.
2) Choose either My Files Only or File from all users on computer.
3) Click Continue when prompted by the User Account Control dialog box.
4) Select the C drive in the drop down list.
5) Click OK and Disk Cleanup will calculate how much free disk space you can gain.
6) Select which File to Delete.
7) Click OK and click Delete Files when Prompted.

Method 2: Disk Defragmenter
1) Click Start > All Programs > Accessories > System Tools > Disk Defragmenter.
2) Click Continue when prompted by the User Account Control dialog box.
3) Click Defragment now.
4) Select which disks to defragment.
(Note: you may also schedule a time for disk defragmentation)
5) Click OK.
(Note: Disk Defragmentation may take a few minutes or a few hours depending on the amount of data on the hard disk)
Method 3 - Check the Hard Disk
1) Click Start > Computer.
2) Right Click the C: drive and click Properties and click the Tools tab.
3) Click Check Now.
4) Click Continue when prompted by the User Account Control dialog box.
5) Click Automatically fix file system errors and Scan for and attempt recovery of bad sectors.
6) Click Start.
7) Click Schedule Disk Check.
8) Restart your computer to begin the disk check on drive C:
(Note: Hard drive disk checks can take a long time, depending on the size of the hard disk and the amount of data)