Okay, close to what the other fellow said.
Enter Safe Mode.
Click the start button (or windows button) on the taskbar.
Go to Run or Click in the search bar if there's one.
Type in Msconfig. A window with several tabs will pop up. Click the one labeled startup.
There will be a list of things with checkboxes. Uncheck all of them EXCEPT your antivirus and firewalls, these should be left on.
What this does is disables everything that loads on start up. This normally helps the computer turn on easier. I suggest running a virus scan in safe mode if you can, as well as defragmenting and running a disk cleanup.
To run disk cleanup or defragmenting:
Open the "My Computer" section
Right click on C: and go to properties.
Go to the Tool tab on this and there you have it. Run a defragmentation by going to defragment and clicking defragment.
There's either disk-cleanup or error-checking, click on whichever one of these you see and it will run its little process, and then most likely ask to continue on restart, which you should immediately do.
Hope this helps.