Excel 2003 Errors w/Win7 Ultimate 64bit? Is there a fix?

The following MS Article might provide the resolution to your problem :):
After you install security update 943985, Excel 2003 calculations return an incorrect result when a Real Time Data source is used in a user-defined Visual Basic for Applications function

All the best! Let us know how it goes.

Have you tried detect and repair?
How to install or repair and reinstall Office 2003 programs and features

How to Repair or Reinstall Office Programs

This procedure detects and repairs problems that are associated with installed Microsoft Office programs, such as registry settings and missing installation files. This procedure does not repair personal files, such as worksheets or templates.

The following two methods describe two ways that you can use the Detect and Repair tool to detect and repair problems with installed programs.

Method 1: Run Detect and Repair on the Help Menu

1. On the Help menu in any Office program, click Detect and Repair.
2. Click Start.

Note To restore program shortcuts to the Start menu, verify that the Restore my shortcuts while repairing check box is selected.

Method 2: Run the Detect and Repair Tool from Add or Remove Programs

1. Click Start, and then click Control Panel.
2. Double-click Add or Remove Programs, click Microsoft Office 2003, and then click Change.
3. In the Microsoft Office 2003 Setup dialog box, click Reinstall or Repair, and then click Next.
4. Click either Reinstall Office or Detect and Repair errors in my Office installation, and then click Install.

Note If you choose to detect and repair problems in your Microsoft Office installation, click to select the Restore my Start Menu Shortcuts check box to rebuild your Microsoft Office shortcuts