Laptop restricted to specific SSD size

I have a laptop (Core2Duo) at this stage it is four years old, I'd like to squeeze another two more years out of it, maybe 2.5. I was planning on doing the last upgrade I could for this laptop which is the HDD, currently has a 5400 (320 GB) it's a larger HDD, but slow. The manual states that the max size I can put in this Laptop is a 128 Gig SSD, the problem is cramming the Windows partition...