How can I re-run an advertised task sequence?

I have a task sequence advertised against my "Reference Computers" collection and want to re-run it to test it again. How can I get this to happen? I know of mandatory task sequences (haven't used on yet) but I'd also like to know how you can re-run a task sequence for a particular PC. If somebody rings and says their system is kaput, I'd like to know how to kick off a task sequence for that computer only, not for an entire collection.

Thanks.