Applicability of Service Tax

Dear All,

Company A is an Indian Company and Company B is a foreign Company and Company C is a company located in India and Company B has supplied Equipment to Company C in India. Erection and comissioning work at Company C is being carried out by Company A onbehalf of Company B.

Now company A wants to raise invoice on Company B in USD, please let me know

1) Whether SErvice Tax is applicable?

2) do we have to pay service tax in advance or any other provisions in the Act?

Thanks & regards

Kiran