I have a WLAN internet connection.
On the server side connection speed of my IP is showing great, but on my PC it's terrible!
I reistalled WinXP trying to cean it from any kind of viruses but nothing changed.
Please help!

Are you sure the signal strength is high enough? I had such problems with my ISP and my laptop, and I was so absent minded that I did not notice the low signal strength. Later on it turned out to be a WiFi modem error so I had to replace it.

Cell phone
Cordless phones
Cell towers (if you are close to one)
Thick walls
Can cause this. If your wireless router or wireless laptop are near any this either move them (not the walls or towers LOL) to a different location. Cell towers can cause this issue and all you have to do is go channel by channel on your router to see which one give you a better signal. You do this by going into the router and seleting a channel in the config. Then make sure your wireless laptop is set to that channel and not set to auto find or detect.

I don't think it's WiFi modem because the router shows that my signal is great, 10 times stronger that I'm receiving on my PC.
I don't understand how these signals can be different, while I'm receiving 7Kb/s, on router graph shows 70Kb/s or even more?!