This would be a decent guess but his RAM isn't even close to full so doubt he's using his pagefile.It means your system is lacking of ram and your operating system is paging like crazy on the hard drive to the virtual memory it has on it.
- Install more memory
- Buy an ssd for Windows only (and don't tell me you can't afford them because you buy 120GB Adata ssd's for less than 20 US dollars).
Buying an ssd and put Windows on that is the better choice than more memory. The combination of both is the best of course, but if you had to choose, better take the ssd because when your computer decides to use the page file again, at least it will come from the ssd, which is much faster than a traditional hard drive.