Firstly, there is no way that Windows update can "update" your computer to Windows XP Professional.
There are 2 versions of XP - Home and Professional, and when it was first installed it was either one or the other and only a new full install of the product could update it.
Depending where you got your PC from it COULD be a pirated copy of windows XP.
If you bought it new from a repuatable company like Dell, IBM etc then it is highly unlikely it is a pirated copy, but if you bought it off a "friend", or someone who builds computers in their spare time it could be pirated.
Microsoft have recently introduced something called Windows Genuine Advantage which is downloaded via Windows update.
This checks if your copy of Windows in genuine or not (by checking well known pirated Windows codes) but there are times it makes mistakes.
http://www.microsoft.com/genuine/ProgramInfo.a spx?displaylang=en&tab=XP
If it IS genuine it should have a small sticker on the side of your PC case, or on the underneath of the laptop, saying what the license code is, something like this:
http://blog.clickonline.org.au/images/oem.jpg
If this sticker is missing then it COULD be pirated.