Go into Device Manager (right click My Computer, select Properties, click Hardware tab). Then go under IDE ATA/ATAPI Controllers, expand that branch. Then double click whichever channel the drive is on (primary or secondary). Then on the Advanced Settings tab under transfer mode change it to 'DMA if available'.
if you can't enable DMA make sure you have the newest intel ATA chipset/driver for your motherboard. some of the DELLS 4100 /early 8100 do NOT have ultra dma and you just need to get the updates from dells site .