2 Answers

malwarebtyes: http://www.malwarebytes.org/lp/malware_lp_form

1) Download the malwarebytes anti-virus software. (it's free)

2) When you're ready to scan: First delete all cookies and close all open applications and programs.

3) Run deep scan (you might want to do this just before you go to bed as the deep scan can take hours.

4) Quarantine everything it finds.

5) Update malwarebytes each and every time you want to scan your computer. This way you can be sure the software is looking for all known viruses. 

How to make a computer faster:

6 ways to speed up your PC http://www.microsoft.com/atwork/maintenance/speed.aspx

Delete Your Browser's History http://www.ehow.com/how_6075_delete-browsers-history.html

Delete Cookies http://www.ehow.com/how_18044_delete-cookies.html 

You can restore your system to a previous time, but this action will take away the programs you have stored after this time. Another way is to reinstall your system, but you need to make a copy of your partitions, which may be a huge project, but I suggest you use partition software to guarantee there will be no damage of your previous data.