Two possible ways:
One, boot from your Mandrake install disk and use it as a rescue dist. Once into Linux, you can reactivate LILO
Two, get a (free) program from WinImage.com called BootPart. There are instructions with it, but basically you run he program from the windows from the winsows dos command prompt with no arguments and it will give a list of bootable partitions with details.
You then run the program again using the number of the partition you want to boot as a command line argument, and a file name in which BootPart will create a binary boot image.
After this check windows boot.ini and make sure that BootPart has added a pointer to the new binary file. If not, add it yourself.
The choice of Linux or windows will now appear in your windows boot loader.