In Google Chrome:
Customise -> Options -> Under the Hood.
Find Change font and language settings, select it and then change the fixed width font from Courier New to the font of your choice.
I can't find an option that says whether this overrides page specified fonts - I suspect it doesn't.
Tools > Options > Content.
In "Fonts & Colours" select Advanced..., then change the value of the "Monospace" font.
To ignore the fonts as specified on the web page uncheck the "Allow pages to choose their own fonts, instead of my selections above" option on the same dialog.
In IE 8:
Tools > Internet Options > General.
Select Fonts and change the value on the right hand column labelled "Plain text font".
The preamble on this dialog states:
The fonts you select here are displayed on the webpages and documents that do not have a specified tetx font.
So I expect that this will not override any fonts set on the page.