I believe there are a number of reasons for not worrying about fragmentation these days. One would be the sheer size of hard drives allow a lot of files to stay together and not be all over the place, two would be the speed of the hard drives, three would be win xp's ability to quickly handle requests and such, four would be faster cpus, and the list likely goes on.
If the computer is set up properly then by far the main impediment these days, in my opinion, is virii, malware, trojan horses, and a host of other words and names. There are a few programs that kind of hog system resources, but aside from that, its the above.
A lot of people feel if their updated a\virus says the system is clean, its clean. Far from it! Today you need an up to date a\virus plus a good number of:
adaware
spybot search and destroy
stinger (from Macafee)
hijack this
microsoft antispyware
popup stopper
cwshredder
Free online system scan from trend-micro - Has saved my butt a few times, found stuff that others miss and repaired them as well! Never harms your pc!
I have all of these and you can google and install them all for free. There is a popup stopper in quite a few of these type of progs, i just use the separate popup stopper but you dont have to.
I bet if you install them and run them you will report back your pc is running a lot faster.
One other thing i do is to go to the run command, type in:
msconfig and once it comes up go to startup and uncheck some of the progs that you dont need to start up all the time. You may have to experiment a bit if you arent used to this, maybe you are, i dont know. If not be careful, you dont want to disable your a\virus or other important progs that should start up with your pc.
This is only what i do and i am only offering suggestions and my own personal opinions. But my computer is running fast.
Good advice + great people = tek-tips