Microsoft has signed a nondisclosure agreement with Yahoo that allows it to take a look at Yahoo’s business, a source familiar with this matter told DealReporter.

Microsoft joins equity firms KKR and TPG Capital, who are also to scruitinising Yahoo’s books and operations, for striking a deal with the struggling internet company. The firms are looking at buying minority stakes in Yahoo of up to 20%, with an objective to eventually take over the whole company.

Yahoo is undergoing a "strategic review" since September after it sacked its CEO Carol Bartz.

Some private equity firms have refrained from entering into confidentiality agreements with Yahoo because of restrictions that would prevent them from forming consortiums, according to Reuters.

Both Microsoft and Yahoo have declined to comment.

Earlier in 2008, Microsoft unsuccessfully tried to buy out Yahoo at a price of $47.5bn before backing out. Yahoo’s stock is considered to be worth less than half of Microsoft’s last offer of $33 per share.