Add a service tag?

I recommend Everest - it creates reports of what's in a machine. You can create your own serial number, as long as you're systematic about it and use unique combinations.
http://www.lavalys.com/index.php?page=product&view=1&subpage=5

Do you mean an asset number that you can see in the bios or a tag that is attached to the computer?
We use a computer name that is linked to the service tag, port number, phone number (VOIP) and cross-linked to our asset tag. If you use an Excel spreadsheet you can query the AD with SQL and import it to Excel.
It is vary easy to search, change and can be AD updated.
If you mean in the Bios, I know Dell has it but you would have to check the website for your Bios to see if you can do it with yours.

If you mean inserting something in BIOS you can get custom BIOS Chips with your Company Logo or anything else from

http://www.biosman.com/

I've found them to be great and very easy to deal with, though I'll admit to never asking for Custom BIOS Chips.

If that isn't what you mean the above answers should do the trick.

Col

Hi

Do you mean adding that information to the My Computer>Properties>General Tab page? If so, try this which I took from: http://www.svrops.com/svrops/documents/reghack.htm#Changing%20your%20Registered%20Name%20and%20Company

"HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows\ CurrentVersion

Select the CurrentVersion key, and you'll see RegisteredOrganization and RegisteredOwner
string values (among many others) in the right pane.

To change either of these values, right-click it and select Modify. Type the new information
on the Value data line and then click OK. Repeat these steps for the other value, if necessary,
then close the Registry Editor."

Good luck