The Windows NT core (XP is really
NT 5.1) is a LOT different from that of the Windows 3.x/9x MS-DOS based operating systems.
Its very hard to transfer advanced skills in 9x over to NT and vice versa, ye have to learn both to be able to play with both.
The first way to find out how clean your PC is running is to check Task Manager (ctrl+alt+del) and see how many processes are listed.
For example, I very rarely have more then ~20 processes running, but then I go to someone's to fix their PC because its running bog slow and see upwards of 50, sometimes even more!
Most of the processes that run on my PC are system processes (ie. processes that the OS needs to run properly). Some have been turned off when tweaking services. But a lot that are on peoples pc's, are things that are running in the background, essentially bloatware, things like Winamp launcher, quicktime updater etc, basically things that developer add to make life easy, but when you have loads of them, they bog the pc down.
If I go into MS Config and look at my startup services, I have just 5 enabled, out of about 45. If all these 45 were starting up every time, then my pc would be slaughtered with regards to memory and cpu usage.
The disk indexing service, which I always recommend to turn off, is actually made to help you find files faster. But the performance gain on searching is not enough to warrant the constant processing every time you ammend the slightest bit of data.
If you have system restore turned on, turn it off, you WILL notice a an improvement in performance pretty much straight away, you will also free up hard drive space. System restore is pretty crap anyway, it is very limited in what it can do, what it can recover and the reliability of restore points.
If you look after the running of ya pc, ye should never need to use it, i havent on my media centre in the 6 years the operating system has been running on it.