HELP - Very Slow Network Speeds !

Hi
My windows 2003 server has always transferred files very fast, moving
files to other machines etc was very good.

I run SQuirrelmail and mercury32 and have found downloading
attachments via the SQ Web interface pretty fast.

Last week my server died and I've replaced the motherboard, cpu,
network card etc...

Since then network transfer speed and SQuirrelmail are very slow..
Moving a 5kb file takes minutes (5mins +)

Downloading a 22mb attachment from SQ Mail - I gave up !!

Any ideas what I need to check ??

Many Thanks