Source: http://lonesysadmin.net/2011/03/08/how-to-set-a-dell-poweredge-service-tag/

Got a Dell PowerEdge that you replaced the motherboard on, and now it doesn’t have a service tag? Reset it with the asset.com tool, which I’ve conveniently packaged as both a floppy disk image and an ISO for those of us in need. They both use the Windows 7 boot disk DOS and asset_A209.com from ftp.dell.com (renamed to asset.com).

[wpdm_file id=50]

[wpdm_file id=51]

Once you boot from these you can set the service tag with:

asset /s <service tag>

You can also set the asset tag with:

asset <asset tag>

Or clear the asset tag with:

asset /d