junk software is typically the issue for why a computer runs slow, so I would start there.
Go to control panel (start menu, then control panel is on the right), and then open up "Programs and Features"
In programs and Features uninstall anything that has the name "Toolbar" in it, and any software that you never use or do not need.
After that is done go back to the start menu and do a search for "msconfig". In MSConfig select the tab listing for 'startup items' and deselect anything that is not hardware or antivirus related.
Hit apply and then go ahead and reset the computer.
If that does not fix things, then you will want to download and run the following tools:
Microsoft Security Essentials: a decent lightweight antivirus. There are better antiviruses on the market, but you typically have to pay for them, or they put a bit of a load on the system. MSE is not perfect, but it is typically adequate protection for most people.
Malwarebytes: The free version only runs when you tell it to. Doing a full scan on your computer can help point out problem software for you so that you know what was causing problems, and what to avoid in the future.
CCleaner: CCleaner is a program which cleans out old useless files that may be slowing things down a bit. But the real gem in CCleaner is the registry cleaner. Removing bad registry settings can remove issues like 'time outs' where software is simply sitting around waiting for something that is not there.
good luck!