[The Fifth Horseman]

You can remove the fan, clean out the dust and apply some sort of grease (like glycerin-based hand creme) to the axle. This worked for me many (MANY) times over.

You can also get a second HDD and a mobile HDD rack, so you can use one of the HDD's just for DOS (your choice which one). It won't take much configuration to get your HDD's set up so that after inserting one HDD into the rack it starts up in DOS mode.

FYI: DOS HDD jumpers set to Master, Windows HDD jumpers set to Slave (assuming both are connected to the same channel), DOS HDD installed in the rack, finally the BIOS configured so that it boots from the first HDD it detects, with the Primary Master HDD set to Autodetect and the Primary Slave set permemently (yeah, I know that's a lot of technobabble) will make it so that after inserting the HDD into the mobile HDD rack the machine will start up in DOS and with the HDD removed it will start up in Windows. Using that very combo myself on an old machine I've got.