There is a way, however, to get it to install, by using these steps.
1) Open Computer and note the drive letter of the flash drive you want to install to. For this how-to, let's say it is drive E:. (This step presumes you have already copied the boot and porteus folders.)
2) Open the Start menu and type cmd.
3) Right-click cmd, click "Run as administrator", type in your password if required, then click Yes or OK.
4) Type your drive letter to switch. In this case, I'd type E: and then press enter.
5) Type cd boot then press enter.
6) Type .\bootinst.bat
7) Read the conditions and press enter. You should now have Porteus on a bootable flash drive.

(**NOTE TO PORTEUS MAINTAINERS:** Most market share is transitioning to Windows 7 from Windows XP; a similar guide, or this one, should be included in the next Porteus release, so new Porteus users can install Porteus to a flash drive.)