Well first off does your friend have the XP cd or is he trying to install 98.
Also it depends if its the xp cd or a restore cd, if its just a regular XP cd, all he has to do is go into the bios and make sure that the BOOT section is set to boot from cd, power down restart and then it will prompt you if you want to format and reinstall on a certain drive, this will erase everything on that drive, and do a clean install. It will put in the default windows drivers and the machine will be up and running.
If you are installing XP from the origional CD, w/o service pack 2 in it it will not reconize the HD's over 130GB I think, but after you install service Pack 2 it will correct the sizes of the drives in the machine.
If it is a restore cd, it is basically the same. But he/she will have it like when it was sent to them from the factory, HP, DELL, etc.
Some of these company's make it so that you will almost have to use the restore CD in the machine instead of a newly purchased copy of XP. Because reinstalling the OP system will need certain drivers that are on the CD. Some are such a pain in the bum, to obtain via the web.
Basically he/she will have to update the drivers regardless, reason being is that the cd is probally a bit old by now and newer releases of the drivers have been out by this time.
Oh BTW the person will probally see a great increase if the person has been installing and deleting programs, and stuff that just clutters the PC till it gets slow, I restore my image about every 6 months.
