From LucasArts' web site:
Why Does The Text Show Up As Solid Boxes?
If you have a video card that uses the Voodoo II or Voodoo 3 chipset, or a Matrox G200 or G400, text will sometimes display as solid blocks when you select multiple units.
3dfx Voodoo II or Voodoo 3 Cards
At the time of release of the game, the DirectX 7 drivers for these cards have graphical problems in the game. When you select multiple units, the text and certain console items will display as solid blocks. When you deselect the units, they will display correctly. We recommend that you check the 3dfx website for updated drivers which may correct these problems.
Matrox G200 or G400
When using a card with these chipsets with the Matrox driver version 5.30, you may experience the following issues:
The arrows in game menus may be missing.
The game may occasionally lockup or crash to the desktop when certain features in the Matrox Powerdesk are enabled.
When you select multiple units, the text and some console items will display as solid blocks. When you deselect the units, they will display correctly.
These issues are corrected by updating to driver version 5.41, which is available on the Matrox website.
When using either of these drivers with the game resolution set to 800x600, you may see a square of corruption that sticks to the cursor. This will continue after exiting game until the computer is rebooted or the desktop resolution is changed.