Windows 7: reinstall of win7 causes black screen and not genuine warning

Recently my PC has begun to work painfully slowly (40+mins to get to the desktop from sleep mode). I think it started happening after the power had gone out so i assumed there was some corrupt windows files. Did a sfc scan and it returned no problems. Tried to run startup repair and still no problems so I tried rolling back to a system restore point but then explorer.exe stopped running automatically and i couldnt bring up task manager or anything along those lines. I opened it in safe mode and checked that the shell part had only the value explorer.exe and it was all fine. Gave up and did a fresh reinstall of windows but now it gets just past the Starting Windows screen only to be confronted by a blank black screen with only the mouse. I left it for a couple hours to see if maybe it was still installing but all that happened was a Windows is not genuine tag in the bottom right corner (my copy is genuine). Tried booting in safe mode but i get an error that says that Windows is in the middle of installing and cant be completed in safe mode and that I should restart my computer and allow install to finish. Any suggestions? Could this be a hardware problem? Could the power out have fried something on the inside?

Try the repairs including hardware tests for Troubleshooting Windows 7 Failure to Boot leading up to a perfect Clean Reinstall following the best practices in Clean Reinstall - Factory OEM Windows 7 .

I just finished another reinstall and its worked this time. The first mustve just been a bit dodgy or something. Although, I still have an incredibly slow pc. Right clicking on the desktop took around a minute for the drop down bar to come down. Does this make it definitly a hardware issues?

Have you installed all of the Windows Updates including optional Updates which should have delivered newer drivers?

Are you running with the drivers given by the installer and these updates?

If so try installing the chipset from the PC or mobo maker's Support Downloads webpage, along with the Display driver.

See if this helps performance.

If not work through the Troubleshooting Steps for Windows 7

ahhh, 122 important updates...I should get onto those