One of two things will happen .... First check your laptop to see if it had XP Retail or XP OEM installed on it .. you can usually just go to 'My Computer, Properties"
it should bring up a box where you can select a whole range of settings & on the first page, it will tell you what os & service pack is installed
Now for the tricky part ... You can not upgrade XP Home OEM to XP Pro Retail or vice versa ... you need the same type .. OEM Home to OEM PRO
or XP Home Retail to XP PRO Retail ... unless you use a key changer utility to change the key to the new os installed or upgraded to .. nothing
illegal about the key changer as you will be using a Legit Paid Key
Or if no software is installed, just let the new xp pro cd do a full format and install ... however, you should have your motherboard & all other drivers
downloaded & saved to a cd .. you will likely need them
Also, if the laptop currenntly has SP2 installed & your new copy of XP Pro DOES NOT HAVE SP2 .. then it Will Not Let You Update
You would need to make a Slipstreamed XP Pro CD With SP2 ... use Autostreamer to make it
See here for how to change the key if needed:
http://support.microsoft.com/kb/328874/