Malious internet threats plague everyone these days, and they are unstoppable. Each day of the week more and more threats are detected and ofcourse as the internet gets more advanced so do the type of threats we face. Hackers, spam, spyware, adware, viruses to name but a few are all waiting to steal your personal information, credit/debit card numbers etc, and phishing is also becoming more advanced by the day. if your pc is running slower than usual its more than likely you have picked up some form of virus or spyware. prevention is always the best practice but still most people think that one antispyware program is ok, but thats not always the case being as some pick up on infections that others don't.
Here are our recommendations:

1 upto date Antivirus program

1 upto date Firewall

Atleast 3 upto Antispyware programs

If your pc continues to run slowly try a registry cleaner and junkfile cleaner as lots of junk ends up on your system, such as when you add and remove programs, temperary files, registry keys etc, all build up and slow your pc down.
Plus don't forget to defrag your drives frequently, as files and directories are created, deleted and moved around your computer pieces of them end up in different memory locations on your harddrives, thus it takes your pc alot longer to find the pieces and open the file, this affects the pc as a whole in time.
Defragmentation puts the pieces back together and improves your computers performance incredebly