Beginning in 2006, Microsoft says it will ship with its operating system and other software products seven brand new typefaces created especially for extended on-screen reading.

The new ClearType Font Collection incorporates improved ClearType and OpenType technologies, and a boatload of research, to improve the structure and the clarity of the letter forms. Basically, that means text will be easier to read because the letters and words won't be as soft and mushy looking.

The Microsoft collection includes two serif, four sans serif, and a monospaced face for use in programming environments. They are intended to be text typefaces as opposed to display faces that are used in larger sizes for headlines. Some of the new fonts are suitable for print as well as on-screen applications.

Suggested substitutions:

Tahoma -> Segoe UI
Courier -> Consolas
Arial/Verdana -> Calibri
Times New Roman -> Cambria

Read more:
 
http://www.microsoft.com/typography/ClearTypeInfo.mspx 
http://www.adobe.co.uk/type/opentype/main.html