Ive got a wireless router.  
Into that router I have 2 normal LAN cables connected.
1.- LAN-cable goes to a printer
2.- LAN-cable goes to a stationary PC
 
 
In addition I have a laptop that connects "wirelessly" to the same router mentioned above.
 
Every time I turn the stationary PC or/and printer ON the laptop "looses" temporarily the connection to the internet.
So in short words the wireless connection interrupts each time a system connected by cable is turned ON.
 
The wireless interruption lasts some seconds, and then the laptop re-establishes the wireless connection automatically.
I'm wondering if it's possible to avoid these interruptions, cause their really annoying. Is this hardware related? Is there a setting that can to be changed in order fix this issue?
 
 
Any ideas would be greatly appreciated!

Assuming the desktop PC and/or printer don't also have a wireless card in them that is interfering, that sounds like a power surge is knocking it out, and it has to go through a reset cycle.  Make sure the router is plugged into a surge protector, or even a UPS.
If the printer is a laser printer, do NOT plug it into a UPS; the fuser uses a heating element which will likely exceed its rating.
I had one that would drop wireless connections every time my 2.4GHz cordless phone was used (cheap Uniden), but going to a 5.8GHZ model fixed that.

Yup my Wireless Router is connected to a surge protector. The printer runs with ink. Neither the stationary PC or the printer have wireless capabilities right now.

The router's adapter (AC power brick) may be marginal.  You might try another one, or at the very least, for testing purposes, plug it in to a different circuit in your house, even if you need an extension cord to reach.
Check the output on it (volts and amps, and plug size and polarity) and find a replacement if possible.
 
Is everything properly grounded?

Almost sounds like you are pulling another duplicate IP from the cable when its activated.  Maybe check your IP addresses prior to connection, then also afterward on each one.  There is a chance that both have had the same IP in the past; most Windows machines will attempt to re-aquire the same DHCP address they have had in the past.  If not, they will get a new one.  
 
But also, your router should have this in its table as assigned and not even allow it to connect and instead assign another IP.  Maybe it's allowing to briefly connect, it sees a duplicate IP, then re-establishes the wireless and assigns another IP to the desktop.

On the stationary PC in Windows XP I've set the IP-adress to be 192.168.1.150.  
The laptop uses the first available IP-adress which is 192.168.1.101.
Maybe I should assign a specific IP on the laptop as well?
 
On the laptop: Actually Windows XP doesn't tell me that the connection to the wireless router gets lost. The way I notice that I had an interruption is for example while playing a multiplayer flash-based game in the browser. In order to continue playing the game I need to press the refresh button in the browser.

Yeah, you could set the IP on the wireless adapter as well.  I am guessing the problem is when the PC that comes online tries to pull an IP that is already in use by the WiFi adapter; so you could set it as well.  Typically, DHCP assigns IPs in natural number progression, so setting the LAN PC might be enough in itself...especially at such a higher number in the octet than 101, 102, etc...
 
Also, you can try pinging your laptop IP from another PC while you switch on the other LAN PC.  PING 192.168.1.101 -t (or whatever your wireless IP is) and see if it starts dropping during the boot process of the other PC (or after an ipconfig /release  and /renew on the LAN PC.  This might give you confirmation of the IP duplication issue...but only if you reset all NICs to DHCP and not statically assigned.  
 
By setting them static...did this resolve your issue?

Weird...duplicate post...

It is also possible that your broadband modem is resetting.  Do either the router or modem have indicator lights that indicate when they are rebooting, or otherwise not ready?