If you are experiencing slow computer response or are unable to download from the internet (especially anti-virus software), you are most likely infected with mal-ware (malicious software). This can occur by landing on nefarious websites and clicking on an area that secretly installs some code on your computer. Here's a generic correction you might want to try to regain control of your computer and send those mal-ware programs to the etherworld.
First, you'll need to go to an uninfected computer and go to www.downloads.com
(This is a C-Net sponsored site and should be safe to download from). In the search on the site, type in MalwareBytes. This is an excellent free program that does a wonderful job of getting rid of most of the mal-ware out there. Download the program onto a USB drive or burn it to a CD.
Now, you'll want to boot up the infected computer into safe mode (hit F8 repeatedly when starting the computer until you are presented with a boot menu). When you get the boot menu, select Safe Mode with Networking. Once in Safe Mode, you'll want to install the malwarebytes program you just downloaded. Install and run the software (allowing it to update if it can). The software will detect and remove most of your problems (especially if it was able to update). Be sure to have the software delete all infections once it is done.
Now, reboot the computer normally and run a full scan with MalwareBytes again (if it was unable to update because malware was blocking it, update it first). Remove all infections (if any) again and reboot.
You'll probably notice a dramatic difference in the performance of your PC once you've accomplished this task. If you found the program useful, consider purchasing it (I'm not affiliated with the program in anyway, I've just found it quite useful and would like to see it continue to be supported).
There are other good programs out there as well, just be careful as there are several scams that claim to be anti-virus and anti-spyware programs that only cost you and install malware themselves.