Please help! My computer is very slow

starting from several days ago... :(

I did a full system restore and scanned all of my files but it did not improve my computers performance. I do not know much about computers so I am not sure if there is anything else I should try. Any advice? Thanks!

As of right now, the only anti-virus software installed on my computer is the Microsoft Security Essential. I am not sure if a virus is causing the lag in my computers performance. Do you have any other suggestions as to what additional software to install?

Thank you!