Following Mr A's article posted in our Readers News section XPSP2 will limit your max. connections/sec ! Here's a Possible fix thanks to Bink.Nu and Mr A!

This articles from microsoft KB explain how to change the TCP/IP parameters. TCP/IP and NBT Configuration Parameters for Windows XP (Q314053) This is the key to add to modify the maximum number of simultaneous connections

TcpNumConnections

Key: TcpipParameters
Value Type: REG_DWORD - Number
Valid Range: 0 - 0xfffffe
Default: 0xfffffe

Description: This parameter limits the maximum number of connections that TCP can have open simultaneously.

TCP/IP and NBT configuration parameters for Windows XP - (Q314053)