Also, press Ctrl+Shift+Esc. This should bring up your Task Manager. Click the CPU column to sort in Descending order and watch to see which processes are taking up your processor. Also sort by Memory to see the same. I have stopped using many programs, such as certain antivirus or firewall solutions, simply because they idle at 2% or more CPU usage or more than 80 MB of RAM.
Make sure that you also have at least 1500 MB of RAM, 4 GB of total RAM+Pagefile, and 2 GB free Hard Drive space.
Do a disk cleanup, and a disk defragmentation as well.
There are commercial programs available that will diagnose and fix speed issues for you. If you are willing to spend some extra cash, buy one of these AFTER shopping aroung a fair amount.
Of course, if your computer hasn't been reimaged (the operating system completely erased and reloaded) for at least four years and you have put a lot of software on it, you should probably just forget this while process, back up your files, and reimage the computer. When you get your computer back up and running, don't install any and every program onto it. Install only programs that you need and will use, and dont upgrade software to newer versions just because they are shinny and have fancy buttons. Not that I have anything against doing this on newer systems, but 4 years is a long time in computer terms (Though not as long as it used to be.)
Use your favorite monitoring tool to watch cpu and disk. when it freezes you should see a spike in either one of those resources.. then just drill down to the process that's causing it and deal with accordingly. It could be your anti virus so just keep an eye out for that one.