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 (x32010@1)A (x02220@1)C (x32010@1)  B (x2444x@1)G (320003@1)A (x02220@1)  E (022100@1)D (xx0232@1)C (x32010@1) E (022100@1)D (xx0232@1)EE--------------------------------------------------------| (022100@1)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