The following are the first steps that I would take when fixing a slow computer.
Many users, after a few months, get a nice collection of toolbars, adware, spyware, virus and two or even more AV programs, etc. This is the usual cause of 'slow computer'. Yes there are other things that could cause slowness but let's deal with the most common causes first.
If you have more than one AV program this can slow the computer down significantly. A lot of people have Norton Systemworks three month trial and when it runs out they leave it on the computer and then add another AV program like Mcafee. This is not a good idea. Uninstall old AV programs before you add another.
**Remember that you should never have more than one anti virus installed at a time**
First, I would run AdwCleaner that is available on this site. Slow computers often have a lot of toolbars, adware etc. and AdwCleaner will help get rid of most of them.
Mcafee is a very poor choice for an anti-virus. You can replace it with the freeware Microsoft Security Essentials. This would be a good choice for you and much better than Mcafee.
Go to your control panel and under Programs> click 'uninstall a program' and uninstall Mcafee, next install Microsoft Security Esentials and do a scan. I you have a problem uninstalling Mcafee there is a Mcafee removal tool that you can download.
Next, in the control panel, 'uninstall a program'. I would uninstall any toolbars that are left after AdwCleaner does it's thing. All old programs that you don't use should also go.
You can also download and run the freeware 'Malwarebytes Anti-Malware'. The freeware version is an on demand scanner which means you have to open the program and click 'scan' and you should do this once a week or so. One more excellent program is Superantispyware which is also Freeware/On Demand. I think you can get these from this site.
These steps should should improve the speed greatly but if it's still slow after you do the above then we need to check some other things like Memory etc.