Before you reformat, try the Kaspersky TDSSKiller, which can find hidden files, such as rootkits and master boot record viruses. It is available for free at http://support.kaspersky.com/faq/?qid=208283363
on the web. It will work on Windows XP and newer computers. You will have to unzip the file. Just accept the default configuration that it comes with. It is updated weekly and will tell you when a new version is available.
After you run TDSSKiller, try a scan with Malwarebytes free, which is available at http://www.malwarebytes.org/products/malwarebytes_free
on the web. Do a Quick Scan with the defaults that it comes with.
If your computer is still slow, then reformat. Good luck!