Today I updated my nasty Windows Vista installation to Windows 7. Also, I had to resize my C-Partition to use the remaining 350GB (swapped my HDD). To achieve this I did the following (and would recommend this way to anybody with similar needs):

The very (!!!) first thing one should do is making a backup. Skipping this step is a bad idea, except there´s nothing important on your machine. I was using Acronis True Image Home 2010, but any other backup tool you´re familiar with is fine. Closing all applications and stopping all unneeded services (e.g. local instances of mySQL and sort of) is also mandatory.

1. I started Microsoft´s Windows 7 Upgrade Advisor and removed everything it told me (especially Toshibas Bluetooth Stack. It was causing problems with Windows Vista, so I assume it wouldn´t work better in Windows 7).
2. Reboot!
3. I removed crap files and cleaned the registry with Piriform´s CCleaner.
4. Reboot!
5. I created a full backup and had a beer in the meanwhile.
6. Reboot!
7. I used Partition Wizard Business Edition, which is free of charge. It did a great job and resized my partitions without a glitch. However, it needed two reboots (without user interactions). Afterwards I checked my filesystems with the chkdsk command (without /f). See the screenshots below to see how the summary should look like.
8. The last and longest step was the updates. I started it at 11pm, and it finished somewhen at 1.30am. FYI, I´m writing this post on my new Windows!

Summary:
This task was rather time-consuming, but It worked without any problems. Also, my installation is noticeable faster than before. Thanks to Software Assurance I am up2date