Hello
When I am using ARCH (after fresh new install) and try to do "pacman -SU" I get really slow!! (allmost freeze most of the time), I tried download the same files inside my windows XP and it download really FAST (200 kb or more)
So
what can be the problem? and how can I fix it please?

I am not familear with ur distro but try to setup resolv.conf in /etc. that could be the reason...

does the problem occur only with pacman or are other programs also getting slow connection speeds??

Do YOU run a firewall? Router? How do YOU connect to the internet? ADSL, DSL, Cable, or Dial Up...etc? Try turning off some of the services YOUR processor is using,...it seems very busy. Take turns with YOUR applications ...if YOUR multitasking and running more than one at a time. Linux runs some tasks in the background. Identify them and decide if they should continue. It might help to add more memory,...but not always...there is a sweet spot that sometimes can improve performance. It is worth the time to check YOUR Power Supply. YOUR system may be maximized for performance, and the drain is inhibiting that anticipated performance.