Don't see why not. I've a few times loaded airtime to send sms's directly from the pc, it's easier than typing on a cell phone.
If you're on prepaid and you use your data allocation up before month-end, then you start using your airtime on the SIM - the advantage being that the airtime expires in only 2030, whereas a data bundle expires at the end of the month.
Like I've said, I have done it on the Cell C 3G modem... use gsm-ussd in Linux, think you can use the dashboard in windows, not sure as, maybe check with someone who uses windows.