As time passed those antivirus apps, and even a script on a web page can cause this on older single core PCs.
You could try looking at the task manager to see if there is some App at 100% and then you have more tools like Process Explorer but there is NO simple do this cure. You have to dive in.
Over the last year I have some substantial slow downs while accessing the web. When the slow down occurs there is 100% CPU usage as listed under the task manager. iexplore.exe always has the highest memory usage when this happens and the computer stops responding. After a few seconds to minutes the CPU usage drops and computer starts working again. We are using XP Pro with all the latest service pack 3.
This is happening to 5 computers on the network. They are all set up the same. HPD220 MT with Pentium 4 CPU 2.80GHz and 1 BG of ram. They used to work fine. I have paid several different professionals to help with this. Is it time to replace computers? Other than Internet my network works fine. I am told my Internet speeds, modem or network are not the issue.The slow down happened 2 times while posting this message. I am not that computer literate. Any ideas?