IF this had been XP Pro retail or upgrade, you could have purchased a legitimate copy and then we could have walked you through changing the product key
However, since you have a volume license edition (also known as corporate, but MS hates that phrase), you will not be able to purchase a legitimate key--the volume license keys are not available to single users.
You cannot use a different key on this version, since the product ID for your installation will not accept a key other than a VLK key.
Side NOTE--there are "ILLEGAL" keys for the VL edition. Keys that have got out in the "wild" and are readily distributed on the Internet have been flagged as illegal. Legitimate license holders were issued new keys and the procedure for changing them.
Given your sitiuation, I sincerely doubt you need Windows XP Pro. Do you have a windows cd, or did this come preinstalled?