I would suggest that you do a few checks:
first run a spyware check
then an antivirus check
defragment your hard drive
check what services are running on your pc by bring up the task manager (CTRL+ALT+DEL) and looking at the processes tab. Each one will have an amount showing how much of the CPU time is being used. Any that are a high amount, need to investigated as to whether they need to be running. Make a list of them and then do a search on google for the process. This will tell you whether it is essential to be running.
You can also download a startup optimiser which will help to speed up the PC as you may have too many application trying to start at once and overworking the CPU.
Hope that this helps.