The best Microcomputer (PC, Mac) image 
manipulation tool out there (and has been 
forever) is Adobe Photoshop. Though I've spent a 
lot time in it, I've never tried to do what you 
want, but I am reasonably certain a cell scan 
could be made to look like a sketch using the 
right combination of filters and effects.

Ask around, see if someone you know has a copy 
you can play with. If you do any image 
work/manipulation at all, time invested in 
learning photoshop is repayed in spades (plus, it 
is a lot of fun): there is nothing out there 
really even in the same class.

The tracing idea below is really good as well.