I managed to find a registry hack that will change all the fonts everywhere in Windows 7, including the logon screen, system information etc etc. Essentially it redirects Windows to the font file you specify when it's looking for MS Sans Serif, or Segoe UI for example. Thanks to Chris Pirillo of chris.pirillo.com.
In the folowing example I used Constantia but you can choose any font you want.
Important: in the [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts] part, make sure you have the actual filename of the font. You can find it by opening the font folder, double clicking the font you want, and then right click on the font and selecting "Properties". Bold, Italics etc all have slightly different names so make sure you get them right. I would recommend creating a restore point before doing this, just in case.
Copy and paste the following into notebook, change the Constantia, constan.ttf, constanb.ttf etc etc to whatever you want, save as a .reg file and voila!
Code:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes]
"MS Shell Dlg 2"="Constantia"
"MS Shell Dlg"="Constantia"
"MS Sans Serif 8,10,12,14,18,24"="Constantia"
"MS Serif 8,10,12,14,18,24"="Constantia"
"MS Sans Serif"="Constantia"
"System"="Constantia"
"Microsoft Sans Serif"="Constantia"
"Tahoma"="Constantia"
"MS Serif"="Constantia"
"Arial"="Constantia"
"Segoe UI"="Constantia"
"Times New Roman"="Constantia"
"Times"="Constantia"
"Small Fonts"="Constantia"
"Tms Rmn"="Constantia"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts]
"Arial (TrueType)"="constan.ttf"
"Arial Italic (TrueType)"="constani.ttf"
"Arial Bold (TrueType)"="constanb.ttf"
"Arial Bold Italic (TrueType)"="constanz.ttf"
"Times New Roman (TrueType)"="constan.ttf"
"Times New Roman Italic (TrueType)"="constani.ttf"
"Times New Roman Bold (TrueType)"="constanb.ttf"
"Times New Roman Bold Italic (TrueType)"="constanz.ttf"
"Tahoma (TrueType)"="constan.ttf"
"Tahoma Bold (TrueType)"="constanb.ttf"
"Segoe UI (TrueType)"="constan.ttf"
"Segoe UI Bold (TrueType)"="constanb.ttf"
"Segoe UI Italic (TrueType)"="constani.ttf"
"Segoe UI Bold Italic (TrueType)"="constanz.ttf"
[HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\Shell\MuiCache]
"@themeui.dll,-2037"="{Constantia, 8 pt}"
"@themeui.dll,-2038"="{Constantia, 8 pt}"
"@themeui.dll,-2039"="{Constantia, 8 pt}"
"@themeui.dll,-2040"="{Constantia, 8 pt}"
"@themeui.dll,-2041"="{Constantia, 8 pt}"
"@themeui.dll,-2042"="{Constantia, 8 pt}"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontMapper\FamilyDefaults]
"Swiss"="Constantia"
"Roman"="Constantia"
Cheers!
EDIT:I should point out that this will also change the fonts in some, but not all, programs.