The boot process would then be like this:

• load OpenSolaris GRUB from third primary
1. boot OpenSolaris from third primary
2. load Microsoft bootloader from first primary
3. load Microsoft bootloader from second primary
1. boot Windows 7 from C: (as first primary)
2. boot Windows XP from E: (as second primary)
4. boot Ubuntu (several entries for different kernels) from logical

It turned out that when installing Windows 7 over XP, it kept the second primary partition as booting partition and installed its bootloader there. Choosing load Microsoft bootloader from second primary in GRUB got me to the MS bootloader screen, where I was able to choose which Windows I would like to boot into. However, if I went for load Microsoft bootloader from first primary, I was getting this error message:
BOOTMGR is missing
Press Ctrl+Alt+Del to restart
or in German:
BOOTMGR fehlt
Neustart mit Strg+Alt+Entf

It means that there was no MS bootloader (of any sort) located on the first primary partition (obviously because Win7 kept the second primary as boot partition).
I was able to fix this by booting via the second primary partition into Windows 7 and running “bcdboot.exe C:\Windows C:” on the command line with elevated rights. This command would create the file C:\bootmgr as well as the directory C:\Boot (which contains the BCD registry file along with language files).
After that I was perfectly able to choose the entry from GRUB which would load the MS bootloader from the first primary partition.
(As a note: if you found this blog entry without having the same setup and a working Windows 7 or Vista installation, you can achieve the same effect via the Installation-DVDs. Maybe not via bcdboot but bootrec)

The second effect I encountered was, when I was trying to edit the text of the MS bootloader on the second partition (change “Earlier Windows version” into “Windows XP”), which is done via bcdedit.exe, all I found was this error message:
The boot configuration data store could not be opened.
The system cannot find the file specified.
in German:
Der Speicher für die Startkonfigurationsdaten konnte nicht geöffnet werden.
Das System kann die angegebene Datei nicht finden.
The reason for this is, bcdedit looks for its registry file (\Boot\BCD) only on the active partition. (You can check this on this Microsoft site)
Note: BE CAREFUL about the program with which you make a partition active! You can safely do this for example via GRUB or Gparted. I strongly discourage from using diskmgmt.msc (under Windows): it removed Ubuntu’s swap and root partition from the partition table and I had no choice but to reinstall Ubuntu (this might be a bug in the Win 7 RC, but I think it’s rather a bug in the disk management console itself).

So, after some fuss about changing the active partitions back and forth while setting messages for the (different) MS boot loader(s), I renamed (just in case) the folder \Boot and file bootmgr on the second(!) primary partition, leaving just NTLDR on it.
Because Windows XP was booting just fine, I am assuming, that an MS bootsector first looks for bootmgr and if it doesn’t find one, it looks for ntldr. (The reason why I believe this is, if I renamed the file ntldr as well, I got the above message about BOOTMGR missing).

The current setup now looks like this (after making the third primary, OpenSolaris, partition active again):

• load OpenSolaris GRUB from thirrd primary
1. boot OpenSolaris from third primary
2. load Microsoft bootloader from first primary, which boots via bootmgr (into Win7)
3. load Microsoft bootloader from second primary, which boots via ntldr (into WinXP)
4. boot Ubuntu (several entries for different kernels) from logical