I recently bought a computer with windows 8 preinstalled on it, and installed Ubuntu 12.04. It ran well at first, but after some time, it started to slow down. This was more noticeable when I was playing games and changing the volume. I tried reinstalling Ubuntu, but the problem persisted. Windows 8 is running fine. I have an AMD Vision A8 Processor (1.9 GHz) and 8 GB of Memory. I have the same problem with other distributions. Does anyone know why I am having this problem or how to fix the problem?
You are probably running the open source drivers which don't perform well in (some) games. I recommend installing the fglrx drivers, you can read more about this on the ubuntu wiki: https://help.ubuntu.com/community/BinaryDriverHowto/ATI
Windows viruses are largely meaningless on Ubuntu or any other Linux operating system. However, you are still subject to attacks by third parties e.g. malware.
Use a firewall (e.g. gufw) if you're using SSH, FTP etc. and install a plugin on your browser that stops scripts running without your consent.
If you haven't taken steps to secure your PC, that might explain why Ubuntu consistently becomes slow after some time, even when you reinstall it.
Those preventitive steps will cost 10 minutes of your time, and while they may not be what is wrong in your case, it's worth the effort anyway - and it would help with the process of elimination.
This is most likely related to one of two things.
Check the temperature sensors and make sure there sane. I had an HP once that no matter what I did the GPU would overheat and cause issues. If your CPU or GPU is getting to warm Linux will throttle it to maintain stability, then if it gets worse, it will shutdown the box to prevent damage.
Poor Hardware Support
I see this a lot. Check all your hardware, specially your graphics card and sound card against one of these methods. On one of my HPs they cheeped out on the sound card and it would cause all kinds of hang ups when power management kicked in.
One more check
Next time the slow down happens, immediately