Are you able to see the grub prompt.
Grub can help you boot into your windows 7/xp environment and from there you can use various tools and utilites to fix MBR. But let me help you cover how are you going to boot into windows system from Grub. Once on the grub prompt.
type the following.
You need to recollect if on partition the windows installed on your first partition, hence change the number according, the below naming convention can help you identify your partition
Grub designation Meaning Usual designation
(hd0) First hard disk /dev/sda (or /dev/hda)
(hd1) Second hard disk /dev/sdb (or /dev/hdb)
(hd0,0) First hard disk, first partition /dev/sda1 (or /dev/hda1)
(hd1,0) Second hard drive, first partition /dev/sdb1 (or /dev/hdb1)
(hd1,1) Second hard drive, second partition /dev/sdb2 (or /dev/hdb2)
Now after this you would be able to boot to the windows system, from there onwards, you can eaisly fix your mbr using the tools from internet. this post can help.