Hi Sven. Let's see if we can give you some help here. I don't know much about laptops because, although I have just recently got myself one, I am a PC man myself. But anyway, I will help as much as I can.
I have to say that I agree with Kees' post above wholeheartedly. Trying to install XP is no easy matter. But below are some of the things you will need to consider.
I may be asking questions within this post. Hopefully you can answer them for me.
You have a Dell Inspiron 1525 laptop with Windows Vista installed, and you play a lot of games. Did this laptop arrive with Vista already installed?
If so, did it also arrive with the Vista installation CD or DVDs, or is there a Recovery partition that allows you to re-install Vista to it's factory condition, or did Dell's paperwork recommend you create Recovery CDs straight away?
If it did not have Vista pre-installed, what was on it before?
Sorry for all the questions there.
You say the computer is getting slow and you want to install XP Professional onto this laptop without losing all your stuff.
This is difficult. We can help with the system getting slow, but installing XP may be a problem, and especially so without losing all your files.
Downgrading from Vista to XP may not be possible because not all computers nowadays are compatible with XP. The problem is that hardware may not work with the older Operating System, (OS), and finding drivers, special files that hardware and some software needs to run on an OS, for XP may be impossible.
Since this is a Dell you will have a "Service Tag" number that came with the system, and if you visit Dell's web site and enter that service tag number, you may be able to find out if Dell supports downgrading this laptop to XP Pro. If they don't, then you are on your own I'm afraid and you must check all the hardware yourself for compatibility, and for all the drivers you will need. This is no simple task and you are not guaranteed to succeed.
You cannot use the XP Pro CD to change Vista to XP Pro because this is not an upgrade. What you could do is to use the XP CD to delete the Vista partition, reformat it, and then install XP as a fresh install. But that will wipe everything from the hard disk and you will lose all those files.
Make sure you have a plan to re-load Vista if this downgrade to XP fails. if you do not have a plan, then you may be left with a laptop case and no OS installed.
One of the main problems that users came across when installing XP was SATA hard drives. XP did not recognise SATA hard drives during the installation process and the drivers for those type of drives had to be installed from a floppy disk during the install. Since your Dell does not have a floppy disk drive, if may be possible to place those SATA drive drivers onto some USB stick and install those during the XP installation procedure. Without those SATA drivers, the hard disk will not be recognised. It may be that the laptop's BIOS will be able to accept SATA drives, but only you can check the BIOS to see whether that option is there.
If you decide to install XP Pro on this system, be sure to backup all of those files you can't afford to lose beforehand. The install process will delete those files.
I hope that helps. As you can see, the problems are not easy to overcome, but I wish you good fortune.