You can find a list of all fonts that ship with any given Microsoft product on the MS Typography website ("Fonts by Product"):
http://www.microsoft.com/typography/fonts/
WPF 4 requires at least Windows XP SP3 with IE 5.01, so the fonts you can count on are the following:
- Aharoni
- Andalus
- AngsanaUPC
- Angsana New
- Arabic Transparent
- Arial
- Arial Black
- Batang
- BrowalliaUPC
- Browallia New
- Comic Sans MS
- CordiaUPC
- Cordia New
- Courier New
- David
- DFKai-SB
- DilleniaUPC
- Estrangelo Edessa
- EucrosiaUPC
- Fixed Miriam Transparent
- Franklin Gothic
- FrankRuehl
- FreesiaUPC
- Gautami
- Georgia
- Gulim
- Impact
- IrisUPC
- JasmineUPC
- KaiTi
- Kartika
- KodchiangUPC
- Latha
- Levenim MT
- LilyUPC
- Lucida Console
- Lucida Sans
- Lucida Sans Unicode
- Mangal
- Marlett
- Microsoft Sans Serif
- PMingLiU
- Miriam
- Miriam Fixed
- MS Gothic
- MS Mincho
- MV Boli
- Narkisim
- Palatino Linotype
- PMingLiU-ExtB
- Raavi
- Rod
- Shruti
- SimHei
- Simplified Arabic Fixed
- Simplified Arabic Fixed
- SimSun-ExtB
- Sylfaen
- Symbol
- Tahoma
- Times New Roman
- Traditional Arabic
- Trebuchet MS
- Tunga
- Verdana
- Vrinda
- Webdings
- Wingdings
(based on filtering this list)
However, these pages note that some fonts might not be installed. I think Asian scripts are generally not installed unless needed -- perhaps someone else knows the details here?