Computer randomly becomes extremely slow and has to be manually rebooted

As the title says my computer randomly becomes extremely slow and even unresponsive at times when it comes to opening/closing programs and the only to get things working again is to manually reboot the computer. I usually leave my pc running all the time (and have done so with many pc's for years) only rebooting it a few times a week when i update drivers or when it seems a little slow. Recently though after the computer is 24 to 48 hours it suddenly most of the time without warning becomes extremely sluggish with some programs working fine, some taking a lot longer than usually to open, while others will not open at all. This also applies to closing programs so will and some won't. The windows task manager doesn't show a spike in cpu or memory usage.

Somethings i've tried to fix it are not overclocking the gpu or cpu, reinstalling windows 7 several times, different video card drivers, different audio drivers, anti virus programs, different bios version's and different power outlets. I'm honestly at loss as to what to try next software wise, which has brought me to believe it may be something hardware related, but honestly i'm at a loss at the moment. Any suggestions or advice would be greatly appreciated.

Specs
Z77 Extreme4 Motherboard
PSU 500W
Intel Core I5-3570K 3.40 (turbo boosted to 4.0)
8GB RAM
Nvidia Geforce 660 GTX TI 2GB (Lightly Overclock)
Samsung SSD 840 Series (Primary Drive) (Running Windows 7 Home Premium)
TOSHIBA DT01ACA200 (Storage Drive)