"Project Professional 2003 cannot be installed when another instance...is already present"

[setup21]

You receive a "Microsoft Office Project is currently installed" error
message when you try to install.

"Project professional cannot be installed when another instance... is
already present".

Here is the solution:

http://support.microsoft.com/?scid=kb;en-us;889503&spid=2525&sid=225

"RESOLUTION
To resolve this issue, force the Setup program to ignore previous
installations of Project Standard 2003. Then, run the Setup program. To
do this, follow these steps.
1. Put your Project 2003 Standard CD in your CD drive.
2. Click Start, click Run, type cmd in the Open box, and then press
ENTER.
3. At the command prompt, type the letter of your CD drive followed by
a colon ), and then press ENTER. For example, if your CD drive is
drive D, type D:
4. Type the following, and then press ENTER:
setup oem_noprevious=1
5. After the installation is complete, click Start, click Control
Panel, and then click Add or Remove Programs.
6. Under Currently installed programs, click Microsoft Office Project
Standard 2003, and then click Remove.
7. Click Yes to verify that you want to remove Project Standard 2003
from your computer.
8. After Project Standard 2003 is removed from your computer, run the
Project Standard 2003 Setup program without the oem_noprevious=1
command-line option."


That helps completely