Depressing, aren't they!
Depressing I may be, but a laptop with 5Gb free that's painfully slow is NOT going to get any quicker simply by mucking around with the hard drive partitioning.
The only related issue would be disk fragmentation, and I'd expect the OP to be complaining about a massive amount of disk thrashing occurring during mild use if that were much of an issue.
(although to eliminate it from your enquiries a quick defrag would help).
So, it's onto either troubleshooting what's slowing it down (which is most probably 3 years worth of crap that's been installed on it), or biting the bullet and going directly to the store and buying something that's inherently several times faster and running the latest software
(TCO isn't just about business computers y'know!)