Restart your computer hit del (or what ever yours uses) to enter the CMOS - under the boot tab select the first boot device to be the CD Rom - disable the other boot devices if possible or select HDD-0 (main hard drive) to be second etc. If the first boot device is the CD-Rom and you have your WINXP CD in the drive it will boot from the CD Rom. Here are some links to help you:
Insert a Win98 bootdisk. (make sure your Bios is set to boot from A:\) Start your computer.
Choose "Start computer Without CD-ROM support" and press ENTER.
Type fdisk, press ENTER.
You get a message about Large Disk Support, choose YES, to enable it.
Press 1 to create Primary DOS Partition, press ENTER.
It'll ask, "Do you want to use all your space for the PDP, choose YES, press ENTER.
Press Esc. to exit fdisk (on some computers you might have to press Esc twice.
At the Command Prompt, type format C: press ENTER.
You will get the message that all data on C: will be lost, do you want to proceed, choose YES, press ENTER.
When it's finished formatting you will get a message about giving the volume a name, I usually hit return and not enter a name, but you can enter a name up to 11 letters, if you want to.
Restart your computer with the boot disk in.
Choose "Start computer with CD-ROM support, press ENTER.
You will see one of these two lines.
Drive X: = Driver MSCD001
Drive X: = Driver OEMCD001
Where X is the drive letter assigned to your CD-ROM drive.
Insert XP CD and type the following command, X:\setup where X is your CD-ROM drive. Press Enter.
XP should now install on your newly formatted drive.