BTW I am a MCDST <Microsoft certified desktop support technician... You are following my directions of your own free will. I am not responsible for anything that gets lost, stolen or broken, please remember that. I have never had any issue in the past.
If you can get into the machine and it is running slower than dirt it sounds like a virus or other malicious program. Restoring to an earlier point will probably do nothing as most of the time it is these files that are corrupted/comprised 1st.
STEP 1: I would get CCleaner downloaded and ran, it is free (or you can buy the upgrade) I use the free version all the time. Direct link:
http://www.filehippo.com/download_ccleaner .Click the green arrow located in the upper right side of the page.
It is very straight forward to use. keep clicking the run cleaner button until "0 bytes removed" is displayed. Then run the registry cleaner. I would not save when it asks you to because it will most likely be garbage anyways. Then click on the fix all issues/yes option. repeat until no issues are found.
Step 2: Download, install, update and run MBAM also free, found here:
http://www.malwarebytes.org/mbam.php then rerun ccleaner.
Step 3: Repeat step 2 with Spybot S&D free found here:
http://www.safer-networking.org/en/mirrors/index.html then rerun ccleaner.
Step 4: Defragment your hard drive. Most of the time you can right click on it and the option will be there.
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FYI "The Geek Squad" runs spybot and declares it fixed for the $50 you pay them. They only guarantee the system for 48 hours so when it comes back, and it will because they are not computer techs., you will need to bring it back and pay again...
What I need to assist beyond the suggestions above, from you, is a bit more info.
-What operating system (other than windows) are you running? xp home/ xp pro/ vista/ 7 ect.
-How long has this been happening?
-Is there something you can think of that was done (clicked on a pop up, went to a file share site?)
-what web browser do you use? IE/Firefox/Chrome/other?
Hope this helps!