Microsoft, the world's biggest software company, may be preparing for a renewed bid for troubled search engine Yahoo. Microsoft's $44.6 billion bid for Yahoo was rejected in 2008.

Circumstances have changed.

Yahoo has been without a permanent CEO since Carol Bartz was ousted Sept. 5. In her day, Yahoo handed over large elements of its search function to Redmond, Wash.-based Microsoft, which has since introduced its Bing search engine.

Yahoo, in Sunnyvale, Calif., has also hired Goldman Sachs and Allen & Co. for strategic advice as well as Heidrick and Struggles to recruit a new boss.

Now, Microsoft has entered the fray again, signing a non-disclosure agreement, DealReporter disclosed, which could mean it will bid for the entire company again. Previously, TPG and Silver Lake Partners, both private equity giants, signed similar deals.

While Yahoo partner Alibaba Group of China indicated an interest in buying all of Yahoo and claimed to have obtained as much as $2 billion in financing from Singapore state investment fund Temasek Holdings, Yahoo director and co-founder Jerry Yang has told outsiders he'd prefer a PIPE deal, referring to a private investment in public equity.

In a PIPE, outsiders acquire a minority interest in a public company, keeping the stock listing but effectively taking control with a cooperative management.

Microsoft, which reported cash and investments of $57.1  billion last quarter, could easily participate in any Yahoo PIPE. Also, having a minority stake in Yahoo could lead to some amalgamation of its MSN and Yahoo News sites, as well as offer additional customers for Skype, the voice-over-Internet-protocol (VoIP) carrier it just acquired for $8.5 billion.

Yahoo officials have not commented on any moves on strategy. During an Oct. 18 investor call to discuss third-quarter results, Acting CEO Tim Morse refused to discuss future moves, promising details when they are ready.

Yahoo shares were unchanged at $14.96 in late Wednesday trading, while Microsoft shares fell one percent to $24.55. Yahoo's market capitalization is $18.5 billion, compared to Microsoft's $206.5 billion.