Error 633 means something else has control over the device, so when you try to connect it's not able to communicate or use the phone to connect.

Basic Troubleshooting

1. Pull the battery out of the phone with it disconnected from the computer and charger
2. Restart the Computer
3. Test the connection again
4. If still not working, close all running applications. Specifically, any Camera software, syncronization software (Such as Activesync, Hotsync, and Blackberry desktop manager if not using a Blackberry), and also try disabling firewalls and antivirus to verify its not conflicting.
5. Test the connection once more

Advanced Troubleshooting

Disable built in modems:

1. XP - Start > Control Panel > System > Hardware tab > Device Manager
   Vista / 7 - Start > Control Panel > Device Manager
2. Click the plus next to Modems
3. If you see another modem besides you phone such as HD Audio Modem or Conexant Modem, right click it and select Disable
4. Test the connection again to see if this resolves the issue.

Disable All Services and Applications using MSConfig:

1. For Windows XP click Start > Run then type msconfig into the text box then press enter or click Ok
   For Windows Vista / 7 click Start and in the Start Search box type msconfig then press enter on the keyboard
2. On the System Configuration window, click the Startup tab then click Disable all
   
3. Click the Services tab and check the box at the bottom that says Hide all Microsoft services
4. Click Disable all
5. Click Apply, then click OK
   
6. A Message will popup asking to restart now or later
7. Select Restart to restart now or Exit without restart to restart later.
   
8. Once the computer is restarted, test the connection.
9. If the connection works, then there is likely an application or service that is conflicting. If this is the case, it can be a lengthy process of enabling one item at a time to determine which application or service is causing the issue...
10. To return the system to normal, follow the above steps to open the System Configuration utility
11. On the General tab, select Normal Startup then click Apply then Ok
    
12. The Restart Message will popup again, select Restart or Exit without restart to restart later.