Is it just the HD that slows the computer down as it gets older?

Simfish

Or will my computer be just as good as new if I, say, replace the hard drive with a new one?

Sure there's dust build-up in the processor, but its effects are related to heat buildup and I found an alternative way of cooling my PC CPU down to levels it may not have seen even when I got it.

It has other mechanical components that wear down too, but are those effects significant over the space of 7-10 years?

mgb_phys

Nothing wears out or slows down with age - two things happen.
New software does more, especially little GUI ricks that take more proessing power and so make it seem slower.
You build up a collection of crud, little task bar apps that are constantly running that you don't use anymore

NeoDevin

Assuming nothing is actually broken: If you reinstall your original operating system and software, and don't get the latest and greatest version of everything, then it should run just as fast as when you first purchased it.

Borg

The speed of your computer is mainly affected by your processor speed, RAM and the needs of your applications. Of course with processors, faster is better. Increasing the amount of RAM is often a good quick fix to speed up your computer. If you are hearing your hard drive running all of the time, it is possible that you don't have enough RAM and Windows has decided to use your hard drive as a virtual cache. The hard drive is much slower for this task and it's very hard on it as well.

I recently fixed a friend's computer that was using over a Gig of his hard drive as part of his cache. His computer was a year-old dual core and should have been very fast. When it first started, it was fine. But, within a half hour, it would get slower and the hard drive was running constantly. We turned off the virtual cache, the hard drive stopped going beserk and everything was fine.

If the slowness of your computer isn't the RAM, then the next step is to see what is running on your computer. Many common apps like browser toolbars and other addons can be terrible resource hogs. I use a Firefox addon called Firebug for my development work but sometimes I have to restart Firefox because it's using too much memory. I've caught the browser using over a Gig of memory on several occasions (1.6 Gig was the record when it crashed ).