The Fair Trade Commission (FTC) said its "on-site investigation" began at the headquarters building of Microsoft Korea in southern Seoul for a five-day run from Thursday.

The probe came after Daum Communications Corp., a leading South Korean Internet portal, filed a 10 billion won (8.6 million dollars) lawsuit against Microsoft for allegedly violating fair trade rules in April.

Daum had previously filed a complaint with the FTC in 2001, claiming the US software giant unfairly bundled its instant messenger service with its Windows XP platform.

Daum, which competes with Microsft in South Korea's booming instant messaging market, claimed the US firm was abusing its globally-dominant Windows operating system.

"As a routine part of investigations, our investigators looked into documents and interviewed staff at the Microsoft Korea office. (The probe) will last for five days," an FTC spokesman told AFP.

Microsoft Korea spokesman Kwon Chan said his company was "actively cooperating" with FTC investigators.

Microsoft has rejected Daum's anti-trust charge, saying Windows XP users have an option to keep using the instant messenger system as installed or not to use it.

The US firm is also in a anti-trust legal dispute with the European Union which in March fined Microsoft a record 497 million euros (612 million dollars) and ordered changes to the way it sells Windows products.

Microsoft filed an appeal with the EU's top court Tuesday, vowing to overturn the European Union anti-trust ruling.

SPACE.WIRE