i have a computer that i bought with a slot 1 motherboard and a pentium 3 450 in it and it is in a really nice case with lights and the works, it have the normal cables in it and i want to put round cables in it,, but when i went to put them in i noticed that my motherboard has all the pins in the connector and the cables has one blocked,, the connector on the end of the cable looks like it has a plug in it that you can take out so that it will work in my motherboard,, if i take it out will it effect anything(i know that the new motherboards have one pin missing),,, thanks

The pin's there to help folks keep the cables straight, which usually get's folks in trouble when they upgrade anyway. Look at the ribbon and you'll see a red stripe on one side of the cable leading into the connector.. This is pin One. Keep that in mind and look at the plugs on the board. You'll see beside the plugs numbers.. You'll see Pin 1 or simply a "1" on one corner of the plug. This will match up to your little red stripe when you plug it in. The same goes for the hard drive, this is easier. The manufacturers usually have Pin 1 right beside the power connectors on the CDROM and Hard drives. Try to line up the pins with numbers on the hard drive and you'll see a 1 on one end and a 40 on the other.. These are the pinouts for your cable.

You can drill out the plug with a small bit on a dremel at low RPM, preferably on a shaft extension, that motor unit is just too unwieldy for this kind of precision work.