Try this command:
mount /dev/hda1 -t vfat /mnt/dos
Two things:
Firstly, check that you have created the file /mnt/dos. You can do this as such:
mkdir /mnt/dos
Secondly, the part of the command mount (/dev/hda1) relates to the harddisk/partition Windows is on. In case you don't know this, harddisks under Linux are identified in this way:
hd : all harddisk drivers in /dev begin with hd
a : This is variable. The first IDE harddisk will have the letter a, the second b and so fourth
1 : The partition on that drive. Windows is usally on 1 or it complains and won't work.
Hope that helps
Profugus