Double-posting, I’m a real rebel.
If you turn off/down some visual effects, you’ll squeeze a bit more performance out of your machine. As I said above, right-click “My Computer”, click Properties, go to Advanced and under Performance, click Settings.
Right now you’ll probably see “Let Windows choose what’s best for my computer” or “Adjust for best appearance”. I have all of those visual options disabled except for “Use common tasks in folders”, you may want to play around with the settings and see.
Also, hold down your Windows Key and hit R (to open a run prompt). Type in msconfig and go to the startup tab. Disable services you KNOW you don’t need automatically on. If you’re not certain, I’d suggest not messing with it. (when you’re done hit Apply, Close, and “exit without restart”)
I’d suggest defragging but you say you’ve got a good defrag app so I assume you use it (Use it more often, perhaps?).
You may want to clear some temp files. Open up your start menu, go to All Programs, then Accessories, System Tools and select disk cleanup. It’ll take a while to analyze when you first open it. Select some things you’d like to see removed, and remove them. The only option I wouldn’t outright suggest is “Compress old files”. It doesn’t actually remove anything like the other choices, but rather just, well, compresses files that you haven’t used in a long time. In my experience it doesn’t really help much. Your mileage may vary.
(You can manually compress files that you know you don’t plan on using soon. I did that with some big install files as well as movies. Just right-click the item in question, go to properties, near the bottom hit “Advanced” and then select “Compress contents to save disk space”)
Your performance is bottlenecked by your processor (perhaps ram speed). I don’t know what your workplace is like, or your motherboard, but it may be a good idea to go buy a better processor. You don’t have to spend a lot to get better than a P4.