In the SYSTEM32 folder there should be a file called 'OEMINFO.INI' If there isnt,make it using Notepad. ("SYSTEM" directory for 98)

Put the following lines in OEMINFO:

[General]
Manufacturer=(Type here without brackets)
Model=(Type here without brackets)

And thats it! What you typed will appear in the system properties page under "Computer"

To set a picture for the "Manufactured and Supported By:" section,move any picture ending in BMP to the SYSTEM32 folder,then name it "OEMLOGO.BMP". ("SYSTEM" directory for 98)