Boot the system off any decent Linux LiveCD, such as Knoppix,
and see if:
a) The Dell partition is still present, usually at /dev/sda1.
b) The Windows partitions is present, usually at /dev/sda2.
c) The Windows partition is marked as "active". There will
be an * next to the active partition.
Beyond that, does the drive(s) do that in another physical
machine? If yes, then the problem is the data on the drives
or the drives themselves. If no, then the problem is in
the hardware on the machine you're working on (backplane,
scsi cable, controller, memory, cpu, fans, ... _something_)