These are last ditch options
First I’d try an EZ-out. Designed for exactly this but in slightly larger applications - they are a tapered left handed thread tap with a square section on the end. It is possible that they aren’t made small enough. I’ve used them on cars/motorbikes with great sucess.
You will need to find one and then carefully drill a hole down the centre of the stuck screw, using the hole you already have as a guide. Stick the EZ-Out in the drilled hole and try to undo the screw using the square section head of the EZ-Out and a pair of gripping pliers [mole grips in the UK] or a small adjustable spanner. The thread tap will grip the inside of the hole and start to remove the screw. The harder the EZ-out is turned the tighter the grip.
Start looking in good tool/hardware stores or even auto parts stores. Take a spare screw and see if they have something that will be small enough.
If this proves to be impossible then you could simply drill the screw out using a drill that is just smaller that the core of the screw [not the thread, the core]. You will need to drill down through the head and into the core, then grip with pliers and rotate. Most likely the head of the screw will snap off, but sometimes the screw collapses inside the thread and can be removed completely. Either way at least you will be able to get the drive out. [As HotBlack already mentioned, you may then have more sucess in gripping what is left with pliers] If you mount it again, just use 3 screws [if you have a Dell Dimension 4100, then that’s all you can use anyway]
If you attempt either of these methods you will need to be extremely careful as tilting the drill will cause you to drill into the drive or case in an undesireable way and damage them. Also make provision for removing the swarf as you drill, a decent vacuum cleaner should suffice.