Why aren't programs opening on my Windows 7 computer?

Various programs don't seem to be opening on my computer, while others do continue working. Currently running Windows 7. iTunes stopped opening last night so I uninstalled and reinstalled it, but now the iTunes .exe file isn't opening. The mouse shows activity and it shows the program in the taskbar for about a second before it dissapears. WinRAR also doesn't open and neither do Disk Defragmenter or System Restore. Please help. Thank you.