Is my PSU to blame?

[Zanna]

I seem to be having a problem, my pc seems to have started up in slow motion. Takes ages to load up to windows (if I enter BIOS, bios is super slow) and when i open things in windows it slowly sort of appears. Now I've disabled all my overclocks to see if that would help and nothing.
When my pc finally got into windows it gave me a bunch of error messages about how it couldn't load usb devices attached to my pc. Could my psu be to blame?
I'm currently running
Intel I5 2500k at 4.5ghz
16gb Ram
650watt OCZ ZS
AsusP8Z68-V/GEN3 Mobo
Palit 560ti (soon to be replaced with a 780)

 

[Zanna]

While I was having the problem I disconnected all drives, removed the gpu (went to onboard) and reset bios. None of these solved the problem. Ran memtest all night and it brought back no errors.
The thing seems to have fixed itself randomly (restarted after letting it boot into windows) but I'd obviously like to try and figure out what went wrong.