First start your system in safe mode
Remove unused programs
Install and run antispyware software
Free up wasted space with Disk Cleanup tool
Defragment your hard disk drive
Disconnect unused network connections:
Click Start, and then click My Computer.
On the Tools menu, click Disconnect Network Drive.
Select the network drives that you no longer need, and then click OK.
ALSO
Stop Unwanted startup programs:
Start > Run > write msconfig > Click on StartUp tab
Deselect the programs you want to stop
Click Start - Run, type in %temp% and then click OK.
Once that temp folder is open and you can view its contents, select and delete EVERYTHING that's inside it.
It's all junk and is a good place for "nasties" to hide.
If a few files resist being deleted, that's normal. Leave them alone and delete EVERYTHING else.
Repeat the steps with c:\windows\temp
After you're done with both temp folders, restart the computer.