Change the default Windows XP fonts

Use regedit and change the following two keys of MS Shell Dlg 2 and MS Dlg to whatever font you want. Make certain that C:\windows\fonts already contains that font.

I have tested and replaced both with "Sugoe UI".

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes]
"MS Shell Dlg 2"="Tahoma"
"MS Shell Dlg"="Microsoft Sans Serif"