my photo stream is not updating on my computer

First check the my photo stream album on your phone to confirm that the photos appear there. If they don't, the issue is with your phone not adding them to photo stream. If the do, some things you can try on your PC are listed below. After each one, try adding a new photo on your phone, then see if it appears on your PC after a brief delay.

1. Going to control panel>programs>programs and features, right-click on "iCloud" on the programs list and choose Repair.
2. Do the same thing except this time choose Delete, then reinstall the iCloud control panel for Windows from here: http://support.apple.com/kb/DL1455 (or here if you're running Vista: http://support.apple.com/kb/DL1687).
3. Make sure you didn't change the default location to save the photo stream photos on your PC. If you did, sign out on the iCloud control panel, then sign back in and enable photo stream without changing the defaults.