I've found that Windows XP is largely self-tuning. Currently I'm running XP Pro and I get 600K download speeds box stock. So you might not wish to modify the RWIN size if your speed is acceptable.

However, If you do want to change your RWIN setting, I have found that the best starting modification to make is to add an entry in the registry that creates a receive window size of 64240. 

To install this modification, Right-Click on this file WindowsNT2000XP-64240 RWIN.reg and select 'Open'.

Select 'Open' again and you will be asked if your are sure you want to install this information in the registry. Select 'Yes' and than re-boot your system at your convenience.

This file adds the TcpWindowSize entry to the registry and sets a value of 64240. It also adds the Tcp1323Opts entry and sets it to 0. Neither of these entries exist in the unmodified operating system.

To return your Windows XP computer back to the original settings Save This File to your desktop and than right-click on it and select install. This will remove both of the entries and it will be as if you never changed it - once you've re-booted your machine.

For a comprehensive Microsoft KB article outlining the adjustable parameters for Windows XP click this link TCP/IP and NBT Configuration Parameters for Windows XP (Q314053)

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