Here's a start and others can add (this is how I do it, but there might be a better way). I'm assuming you have installed Tortoise CVS according to the instructions found on the Moodle CVS page.
1. Create a folder called moodle.
2. Right click on the folder and choose CVS Checkout and you should see a screen like the image below. Make sure yours looks the same. Click on the revision tab and make sure "Use Head branch" is chosen. As far as I know, this is the branch that 1.5+ is in and is the latest stable code. If this has changed, somebody let me know!
3. Then, to download Moodle just click okay.
4. A new window opens and the download begins. You might see this at the top of the window: cvs checkout: Empty password used - try 'cvs login' with a real password. Just ignore that and be patient, the download should begin shortly.
5. If you want to get something out of contrib, find out what folder it is in first by snooping around the sourceforge page. Then, create another new folder like above, then change the module (it says moodle now) in the bottom of the screen to whatever you want to get including the contrib folder. For example: contrib/the name of the module or block folder.
Whatever you download will include a CVS folder, and the folders should be shaded green. If you want to change the branch, you can right click on a green folder, then mouseover CVS, then choose update special from the list. Then you can change the branch. And there are other options their too.
The files that download are labeled in the CVS download window.
C means the file has changed both in CVS and on your computer and their were conflicts between the two.
M means the file has only changed on your computer.
P means the file has changed only in CVS and your copy was updated to match.
U means the file is new and has been added to your folder.
? means the file only exists on your computer and not in CVS.
Hope this helps