Please Help! Sending SMS message from computer to mobile number.

[Pola]

We are developing application on Win XP
that sends messages according to accured events in our system in the real
time to our clients.
For now we send e-mail messages to our clients using SMTP protocol.
We have to be able in our application to send SMS messages to clients mobile
phones in different countries(USA,Germany etc.)

We want to be able to send close to 10000-100000 SMS messages in month.

Pls help us to find the efficient technology to support SMS?
What API ,Gateways,examples can you reccomend?

thank you in advance,
Pola