Thank you very much for helping me! The whole setup worked nicely now! I was able to create a 20GB hard disk. It's a very tricky installation if you want to create a smaller disk than the default one. 
I'd just like to know if I understand the whole procedure correctly.
If, for example, the whole situation is like this:
free disk space on Macintosh HD: 200GB
used disk space
on WinXP virtual disk: 10GB (of 20GB)
on Win7 virtual disk: 20GB (of 30GB)
Then I will be able to use 170GB on MacintoshHD, right? Even though the virtual disks are 20GB and 30GB 'big'.
If so, will I be able to delete files from the virtual disks and use the gained space in Mac OS? Let's say my virtual disk of 20GB is full and I delete 10GB of data, will there be 10GB more space on the MacintoshHD?
Hopefully I understand this now Because if I do, it's not too complicated at all.
The only thing still left unclear is that I really don't know what happened to the 40GB missing. After reinstalling WinXP with a 20GB virtual hard disk I did not lose any further disk space except the place the virtual machine itself uses. Even after I tried deleting it again and installing it for the 3rd time, there was no further Macintosh HD space missing. I guess these 40GB must have been used by something else.
EDIT:
I also got one more question there: When it comes to RAM, I can change the amount after installing the guest OS, right? Let's say WinXP may use 512MB and once in a while when I need more, will I be able to increase the amount of RAM WinXP may use?