As on Xbox 360, the pair will be available as individual downloadable content packs for GTA IV or in a standalone retail double-pack 'Episode From Liberty City.'
The Lost and Damned hit Xbox 360 in February 2009 with a tale of Libery City biker gang 'The Lost' then nightlife-focused 'Gay Tony' followed in October--an exclusivity period which cost Microsoft $50 million. The PC DLC release was hinted at in November when a GTA IV patch added the achievements from The Lost and Damned.
All will be released on March 30. Pricing has yet to be announced, though for reference the packs cost $20 each on Xbox 360 while Episodes from Liberty City was $40.