You'd have to find a copy of the fixedsys font with the greek script and install it the same way you would install any other font into Windows. mIRC doesn't come with fonts, but uses the fonts that have already been installed on the system that mIRC is being installed on.
From the Windows help system (Windows XP Home)
To add a new font to your computer
Open Fonts in Control Panel.
On the File menu, click Install New Font.
In Drives, click the drive you want.
In Folders, double-click the folder that contains the fonts you want to add.
In List of fonts, click the font you want to add, and then click OK.
To add all of the listed fonts, click Select All, and then click OK.
Notes
To open Fonts, click Start, click Control Panel, and then click Appearance and Themes. Under See Also, click Fonts.
To select more than one font to add, hold down the CTRL key, and then click each of the fonts you want to add.
You can also drag OpenType, TrueType, Type 1, and raster fonts from another location to add them to the Fonts folder. This works only if the font is not already in the Fonts folder.
To add fonts from a network drive without using disk space on your computer, make sure that the Copy fonts to Fonts folder check box, in the Add Fonts dialog box, is clear. This is available only when you install OpenType, TrueType, or raster fonts using the Install New Font option on the File menu.
If you're using a different version of Windows, you'll have to look up how to install fonts.
