Markus;
Repair Installation:
http://www3.telus.net/dandemar/repaxp.htm

I'm not Michael, but here ya go:
"How to Perform a Windows XP Repair Install"
http://michaelstevenstech.com/XPrepairinstall.htm

I'm really sorry Markus, I guess I hit send when I meant to hit paste.
There others have you covered but for the record, here are the instructions:

NOTE, while a repair install should leave your data files intact, if
something goes wrong during the repair install, you may be forced to start
over and do a clean install of XP.  If you don't have your data backed up,
you would lose your data should that eventuality occur.

Assuming your system is set to boot from the CD-ROM drive and you have an
actual XP CD as opposed to a recovery CD, boot with the XP
CD in the drive and perform a repair install as outlined below.  If the
system isn't set to boot from the CD or you are not sure, you need to enter
the system's BIOS.  When you boot the system, the first screen usually has
instructions that if you wish to enter set press a specific key, when you
see that, do so.  Then you will have to navigate to the boot sequence, if
the CD-ROM drive is not first line, set it first in the boot sequence.  Save
your settings and exit with the XP CD in the drive.  The system will reboot.

When the system boots, a few screens into the process you may see a message
instructing you
to hit any key in order to boot from the CD along with a countdown.  When
you see this be sure to
hit a key on the keyboard, if you miss this instruction and the system fails
to boot from the CD, it's too
late, you'll need to reboot and try again.

Once you have pressed a key, setup should begin.  You will see a reference
asking if you need to load special drivers and another notice that if you
wish to begin the ASR (Automatic Recovery Console) depress F2.  Just let
setup run past all of that.  It will continue to load files and drivers.

Then it will bring you to a screen.  Eventually, you will come to a screen
with the option to (1) setup Windows or (2) Repair Windows Installation
using the Recovery console.   ***The selection you want at this screen is
"Setup Windows,"
NOT "Repair Windows Installation.

The first option, to setup Windows is the one you want and requires you to
press enter.  When asked, press F8 to accept the end user agreement.  Setup
will then search for previous versions of Windows.  Upon finding your
version, it will ask if you wish to Repair your current installation or
install fresh.  Press R, that will run a repair installation.  From there
on, follow the screens.

Note, in some cases, you won't receive the repair option, only an option to
reinstall.  We have discovered that sometimes this is caused by damaged
boot.ini file that can be repaired as follows and also note, in the
instructions, "K" refers to the CD drive in which you have placed the XP CD,
replace that drive letter with the appropriate letter on your system, "K" is
simply an example.

Reboot, this time taking the immediate R option (this is the section I told
you to skip above.  In this case, you will need to get to the Recovery
Console to perform the function below), and if the CD letter is say K: give
these commands

COPY K:\i386\ntldr  C:
COPY K:\i386\ntdetect.com  C:
   (two other files needed - just in case)
ATTRIB -H -R -S  C:\boot.ini
DEL C:\boot.ini
BootCfg  /Rebuild

Once you've completed this function, reboot and see if you can access XP as
sometimes, the problem is the damaged boot.ini.  If you still cannot access
XP, then reboot and re-run the repair install instructions at the beginning
of this message.

If you only have a recovery CD, your options are quite limited.  You can
either purchase a retail version of XP which will allow you to perform the
above
among other tools and options it has or you can run your system recovery
routine with the Recovery CD which will likely wipe your drive, deleting all
files but will restore your setup to factory fresh condition.