This is an FYI for any readers who may experience what I experience more and more frequently when servicing XP Professional and Home computers both at work and for friends at home. I sometimes get reports that "my mouse isn't working" or "I was only checking my webmail when my computer froze". By the time I see what is happening the computer often has resumed operation. If not, a restart usually brings it back to normal. All of these computers have been older Pentium 4 computers with between 512MB and 1GB RAM. When I try to duplicate the problem by running the same applications simultaneously (such as a browser, Microsoft Office, antivirus update, etc.) and I check the Task Manager I see that the lockups occur, understandably, when the CPU is running at 100%.
To see if this may be happening on your computer if you start to experience a slowdown right-click the Task Bar and select Task Manager. Then click on the Performance tab and you will see both CPU usage and physical memory usage. If your CPU is running anywhere near 100% consistently your only choices are either to wait until the CPU-intensive task is finished and it releases the CPU or to end one or more tasks that are running. Even if you have enough memory to spare your computer cannot work properly once the CPU can no longer handle any more tasks.
As you can imagine, newer computers come with newer processors that can better meet the demands of modern computing. Pentium 4 CPUs are a minimum for running XP and were once the "latest and greatest" but now you may need to follow my suggestions above when they start to become overloaded with multiple tasks.