Different Text formats using a function

Hi.. i have a string like this :

first line

second line

third line

forth line

fifth line

sixt line

now i need a function to do this :

put <b> </b> around the first line

put <i> </i> around the second line

put <br> instead of the third line which is a crriage returne

and then loop the same thing for the rest..

put <b> </b> around the forth line

put <i> </i> around the fifth line

put <br> on the sixth line... and so on...

how can i do that using a function in VBScript?!?!

tanx sooo much