I purchased a 'home made' pc from someone which had XP Professional installed, after about a year of using the pc, I recently did a windows update, now I have an icon on the bottom right as well as a message when windows starts up asking me to purchase a copy of XP. I am assuming this is some kind of serial key issue, is there anyway to get around these annoying messages etc?.

I have no idea what the serial key is.

I purchased a 'home made' pc from someone which had XP Professional installed, after about a year of using the pc, I recently did a windows update, now I have an icon on the bottom right as well as a message when windows starts up asking me to purchase a copy of XP. I am assuming this is some kind of serial key issue, is there anyway to get around these annoying messages etc?.

I have no idea what the serial key is.
It appears that the copy of Windows installed on your computer is counterfeit.
It does not use a genuine CD Key.
There are quite a few illegal solutions floating around the Internet, but unfortunately these are temporary fixes as Microsoft will address this counterfeit issue every month with Windows Update. You would be left facing the same situation every month.

You would need to purchase a legal copy of Microsoft Windows XP or contact Microsoft about acquiring a legitimate CD Key.

Your solution is not a simple one.

Most likely a Volume License (Corporate) copy of Windows XP was installed on your system and Microsoft does not issue VLK license to individuals.

You would then be forced to purchase a Retail or OEM copy of Windows in order to get a valid CD Key.
Unfortunately, your VLK copy of Windows XP will not accept CD Keys from Retail or OEM.
Your only choice here is a fresh installation of Windows from a legal Retail or OEM disc.

If you should contact Microsoft and provide them with the source of the counterfeit copy, they might reimburse you with a legal copy of Windows XP as you were a victim here. They will then persue legal actions against the source you provided.

For more information do a search for Windows Genuine Advantage or WGA.