I can't say specifics if you are using lilo, which I think is Mandrake's default bootloader, but if you are using grub I can try and help you out.
The only info I can give you at this point is to make sure you make a linux boot disk before to try to fix your Windows intall (just saying it again because I forgot one time... hehe...

)
Since you are using Mandrake, after you reboot, the only thing you should have to do is to load up the mandrake controll panel, go to the boot section, and setup the bootloader again there.
I've never done that, but I have used it to switch between bootloader's before, so I assume it will work.
Also, if you're just starting to use linux, you may want to check out
Fedora Core. My bro installed it on one of our systems and said that it's been working really well.
Cheers!