Instead of replacing the Hard Drive why not just get another one and set it as a slave. Or **** if you got money get 3. Also maybe get another gb or ram if it can hold it.
If you comp is running slower than usual theres a good chance its just spyware and adware. Download adaware se from
www.lavasoftusa.com its free and it rocks. Check your startup items and dont use IE use mozilla firefox. Make sure your comp is secure with a firewall (i suggest kerio personal firewall), make sure all the drivers dont have updates, perform windows updates. That is if its running slower than usual.