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)

 

[ExRayTed]

Whilst the PSU isn't a 'quality' unit, I'm not sure the symptoms you've described can be attributed to it.

The fact that you're having issues in the BIOS certainly points to hardware failure of some description and the errors once in Windows suggests it is the hard drive.

I'd firstly ensure all the cables are connected correctly. Then I'd reset the BIOS, just to be sure. If you're running an SSD make sure to enable AHCI mode in the BIOS once reset. If that doesn't fix the issue then I'd be disconnecting the hard drive and see if the problems persist within the BIOS. If they do, disconnect any optical drives and try again. Then try running each stick of RAM on its own.

If all that fails report back and we can go from there

 

[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.

 

[ExRayTed]

Glad you got it sorted. Just a heads-up, running memtest does not guarantee that the sticks of ram are not problem free. If the problem resurfaces I'd still recommend trying each stick on its own.