Hi
There is so much that can be wrong with it. Where to begin...
Do you have a proper antivirus on your computer that is up to date? Scanning your computer with malware bytes is a good free indication of how clean it is, just make sure to have backups in case u get the rare problem.
Have u installed any hardware or software lately? or are u using it in a different USB port every time, found that if u swap usb ports often that it corrupts the drivers, where u need to uninstall all traces of the drivers to get it working.
Some times it tries to work only as a CD Rom, and disabling it in device manager can help(but only the CD Rom The modem creates).
Try doing a firmware update on it, found this problem allot on 3G modems and windows xp.. normally I try the firmware for the provider that the 3g is for first.. if that doesn't work i get the firmware from the manufacturer of the card.. like with the hauwi models the standderd firmware works the best. If u are not sure about how to do this, there is a few guides, I have also found that both MTN and Vodacom staff is very helpful if they have a tech that knows how this works.
Hope this helps. If not, please let me know
