If you want icons for your dock the best place is to look around Design Technica for PNG's. But also look at the Mac Icon sites as they generally have the best icons. You can convert the icons into PNG's and use them for your dock. (As Mac icons are very similar to PNG's)
Xicons is the best one (guide on their site too on converting icons)
They also have a new winxp section but it is still very new and only a few icon sets are in it.
You'll need stuffit expander to read the mac archives and extract the icons.
Also an icon editing program to convert the icon into a PNG.
Axialis Icon workshop is the best and easiest to use, you can download a trial version.Axialis Link
All you have to do is download the file,
rename it to *.icns
load it into the program,
select "file" and "export"
select "PNG with transparency"
and your done.
(windows icons dont have to be renamed .icns,only mac ones)
(to make the job easier converting icons open a command prompt in the folder and type "rename . *.icns" so it will rename all the extracted icons in one go)
This site has some flash based tutorials on how to extract/configure and convert the icons but they didn't work for me I think my firewall blocked them thinking they were adds.
(Forgot to mention the most current version of Stuffit v8 does not require any special settings to extract the files from mac archives, previous versions did. However if you see a file just called "Icon" don't extract that one as it will stop the extracton procedure for all the files. Not sure what it is as the tutorials have not been updated to reflect the newer version of Stuffit. It didnt happen in the older versions though)