I have two 100G hard drives. Drive C is almost full and Drive D is almost empty. I would like to find out how to link the two HD so that the files seamlessly flow into Drive D as C fills up.  I RUN Vista on a 64 BIT HP pavilion laptop, processor 2Ghz, 2 GB RAM. How do I link the 2 hard drives so that they work as if they are one?I've never DONE this so I can't comment on how effective it is, but I found this after a quick search http://www.computing.net/answers/windows-xp/combine-2-hard-drives-into-1/86177.html. So it appears that it is possible, using dynamic disk management. Or you could set up a raid array.
Be aware that doing this may minimise your ability to recover data from either drive.

Good Luck.I'd highly recommend the RAID option, especially if you can get a third 100GB drive (for one of the RAID levels with REDUNDANCY (RAID-5? I don't remember). Dynamic disks are... well, DIFFICULT to manage. everything works alright as long as all disks remain in tip top shape, but if you lose a single disk you lose all the data in all the disks (if I remember correctly). At least with RAID, you have a protection mechanism.

Also, with Dynamic disks, it's a software mechanism implemented by windows- for RAID you can often configure a hardware solution.