I made one using a 172x114 pixel bitmap. Change the name to \"oemlogo.bmp\". Copy this into the WINNTSystem32 folder. You can also create a system info file. Start with a plain text file and rename it \"oeminfo.ini\". It also goes into the System32 folder. I believe for Win9x it goes in either WindowsSystem or WindowsSystem32.
Here is the format:
[General]
Manufacturer=\"Joe Blow Computers\"
Model=\"Turbo\"
[Support Information]
Line1=\"\"
Line2=\"For Technical Support contact:\"
Line3=\"\"
Line4=\"Kiss My:\"
Line5=\"123 Maple Street\"
Line6=\"Anytown, USA\"
Line7=\"800-PLEASEHOLD\"
Line8=\"\"