Here's your manual in case you don't have it:
http://support.dell.com/support/topics/global.aspx/support/dsn/en/document?c=us&dl=false&l=en&s=gen&docid=45FAC9DF3456B193E040A68F5A285493&doclang=en&cs=

See page 82 for system setup, but I do not see any USB device listed under bootable devices in system setup. Sounds like you'll need to replace that CD drive. You need an IDE CD or DVD drive. Here are the instructions for replacing that drive:
http://support.dell.com/support/edocs/systems/dim1100/EN/SM/parts.htm#wp1088942

You should not have any trouble finding a used CD drive for under $10. Don't forget to set the drive jumper to cable select.