You need some kind of bootable media.
On a working pc download and burn to cd PW free boot cd. Partition Wizard Bootable CD allows user to manage partition directly with partition manager bootable CD.
Boot it up on the sick machine.
If you have 100mb partition, rt click it set as Active, ok , apply.
If you don't
have 100mb partition - then try setting the main windows partiton Active instead - if that doesn't do it, set the manufacturers recovery partition Active.