first thing firs have a scan at it with ur antivirus software i recomend avast if u want free one or norton
then a goood scan with antispyware software like
Spyboot S&D
then run msconfig on run command and on the startup tap see if tere is any unknown process to u and uncheck it
Restart and this should at least for sure give u a 30% faster pc
If thing are still not so good
recomended a repair of ur os with its cd and after that open antivirus sw run to check for anyviru since ur registry will be clean at the moment delete if any run all the other appz u had they will also rewrite themselfes,,defragment and make a clean up and garanted u will have a 70% faster pc
let me know what didi u do or give me more info on ur pc i will write more advice and help for u
Cheers