This was the case with my mom-in-law's PC. I ran CCleaner Portable on it, it had almost one and a half GB of temp internet files alone. Not counting the temp system files. She had that PC for 6 years & never used the Disc Cleanup tool, nor the defrag tool. Didn't even know they were there, nor what they were for.
It was also running at 64% fragmentation. All of these things leads to performance loss.
Last night, on my notebook, I was trying to see my folding stats on Extreme Overclocking, and I had trouble getting the page to load. A reboot cleared it up. It had been over a month since it had last been rebooted, over 40 days in fact. I really didn't want to do it, but past experience showed me that sometimes a simple reboot fixes things.
But that reboot cleared things up, and I was able to browse as usual. Hopefully that raises my points too, they were off about 2,000 from the previous 2 weeks, although I did complete more WU's.
Cat