Mr. Frisk is of the opionion that Windows has no issues. All those questions in Yahoo Answers about Windows issues with black screen, blue screen of death, missing DLL, etc. are all fake.
There are several versions of OS X. Which is yours? My answer is for OS 10.5.8.
Follow these steps in this order:
-- Disconnect everything except the power cord.
-- Restart and hold the shift key for 30 seconds. NOTE: This will cause a 1-2 minute delay at the spinning gear icon, so just wait to see if it continues. If that helps, go to "Shift" below. If not, read on...
-- Boot to the OS X DVD. Go past the language choice, and open Disk Utility. Repair the HDD. If problems found, repair again.
-- If no problems found, quit Disk Utility, quit the installer, and restart while holding ⌘ and the S key for 30 seconds. NOTE: This next group of commands are for those who either hate to reinstall or don't have a reinstall DVD. Any mistake in typing could damage the system files or your saved documents.
Type:
/sbin/mount -uw /
Press Return.
Type :
sudo rm -r /Library/Caches
Press Return.
Type your admin password (you will see nothing but the key entries will have the effect).
Press Return.
Type:
sudo rm -r /System/Library/Caches
Press Return.
Type:
sudo mv /Library/Fonts /Library/Fonts-old
Press Return
Type:
sudo mv /Library/Preferences /Library/Preferences-old
Press Return.
Type:
reboot
Press Return.
If that doesn't help, boot to the OS X install DVD, go past the language screen, and...
-- for OS 10.5: go forward to choose "Options", and then "Archive and install", but put a check for "Preserve Users and network settings".
-- for OS 10.6: Reinstall OS X.
Shift -- Restart again not holding shift. If that works, you're done. If not, troubleshoot Login Items.