Don't delete older security patches. Subsequent updates often run on top of older ones. To start deleting them could corrupt critical operating system files and lower your computer's overall security.
Regarding the slowness of your computer, it could be a variety of factors. You should first run your security products (antispyware and antivirus) to ensure you haven't any malware. You can then check to see if you have too many programs running that is draining your system resources (RAM). Also you can run a disk cleanup, chkdsk, and defrag your computer as well as clearing out your browser cache (cookies, history).
If you determine that it is a system resource issue, you can see if your computer can handle more RAM by going to Crucial.com and running a scan.
http://www.crucial.com/index.asp
It's important to note that everything takes RAM. Not only security products (such as active anti-spyware, anti-virus and firewall) eat up memory but your operating system, active browser windows, running applications (such as Word, Gaming, Messenger, Photoshop, etc), graphic card and malware all compete for system resources.