This is the tab for Track 17 from the Half Life 2 OST.
It is also known as Path to Borealis.
This is only the main theme, played on the A Minor scale.

   C  A  C    B  G  A    E  D  C   E  D  E
E--------------------------------------------------------|
B--------------------------------------------------------|
G--5-----5----4------------------------------------------|
D-----7----------5--7------------------------------------|
A------------------------7--5------7--5--7---------------|
E------------------------------8-------------------------|

Kelly Bailey last.fm page:
http://www.last.fm/music/Kelly%2520Bailey