How to download Bootcamp drivers without Bootcamp assistant?
Yes, It is possible to download the Bootcamp drivers outside of the Bootcamp tool:

Updated: 


timothy-sutton's answer will make your life so much easier, go there first
For Windows 7 through 10, Apple have published a table of 'which download you need for which model' at https://support.apple.com/kb/HT5634. Win 7 download might work for Vista.
For WinXP, see robmathers' answer.
The hard way which follows here, taken from 
http://www.cafe-encounter.net/p682/download-bootcamp-drivers is your fallback if you want or need to work it from scratch.


Download from Apple the http://swscan.apple.com/content/catalogs/others/index-lion-snowleopard-leopard.merged-1.sucatalog file. Don’t double-click it.
Open the index-lion-snowleopard-leopard.merged-1.sucatalog file in a text editor or word processor.
Search for each occurrence — as at August 2012 there were 6 — of BootCampESD.pkg. For instance, the one I needed is http://swcdn.apple.com/content/downloads/33/54/041-2011/pRtCDYcWShMLxFggy3TzFzmfnnWQNFQBfJ/BootCampESD.pkg
Notice in each such URL, the /041-2011/ or similar /041-XXXXX/ bit of it.
Below each such occurrence, notice the URL for a file with the same 041-XXXXX in it and ending in English.dist, e.g. 041-2011.English.dist
Paste the URL for each such English.dist file into your browser and open that url. Here’s a list of them:


http://swcdn.apple.com/content/downloads/11/05/041-0925/g27es04pw9re5ggrfp3vuf8ew6r53asfz8/041-0925.English.dist
http://swcdn.apple.com/content/downloads/53/34/041-4776/TpFRDjFyRJjSvtyj2vFsPyx2bH2NSYvwN9/041-4776.English.dist
http://swcdn.apple.com/content/downloads/55/51/041-3891/se4uhpqng48t842cdsosqh28lft54fmswl/041-3891.English.dist
http://swcdn.apple.com/content/downloads/32/15/041-4775/hgMfrNHd9W8W7d8n4JyqSFLksk5TYr2HXN/041-4775.English.dist
http://swcdn.apple.com/content/downloads/32/33/041-2303/W5jKtsQhhMgMXs3TKhPDqLXrpXyY2HdXMN/041-2303.English.dist
http://swcdn.apple.com/content/downloads/33/54/041-2011/pRtCDYcWShMLxFggy3TzFzmfnnWQNFQBfJ/041-2011.English.dist

Search for the Model Identifier for your Mac. For instance MacBookPro5,2 or Macmini4,1.


For instance the 041-2011 file contains these models: MacBook2,1 MacBook3,1 MacBook4,1 MacBook5,1 MacBook5,2 MacBook5,3 MacBook6,1 MacBook7,1 MacBookAir1,1 MacBookAir2,1 MacBookAir3,1 MacBookAir3,2 MacBookPro2,1 MacBookPro2,2 MacBookPro3,1 MacBookPro4,1 MacBookPro5,1 MacBookPro5,2 MacBookPro5,3 MacBookPro5,4 MacBookPro5,5 MacBookPro6,1 MacBookPro6,2 MacBookPro7,1 MacBookPro8,1 MacBookPro8,2 MacBookPro8,3 MacPro1,1 MacPro2,1 MacPro3,1 MacPro4,1 MacPro5,1 Macmini2,1 Macmini3,1 Macmini4,1 iMac5,1 iMac6,1 iMac7,1 iMac8,1 iMac9,1 iMac10,1 iMac11,1 iMac11,2 iMac11,3 iMac12,1 iMac12,2
How do you know your Model Identifier? Open System Information, and look in the Hardware Overview section. i.e. click Apple menu -> About this Mac -> More Info… -> Report -> Hardware -> and now read down the Hardware Overview looking for “Model Identifier:”

Having found your 041-XXXXX number, download the BootCampESD.pkg url that has your number in it. Again, here’s a list of them:


http://swcdn.apple.com/content/downloads/55/51/041-3891/se4uhpqng48t842cdsosqh28lft54fmswl/BootCampESD.pkg
http://swcdn.apple.com/content/downloads/32/15/041-4775/hgMfrNHd9W8W7d8n4JyqSFLksk5TYr2HXN/BootCampESD.pkg
http://swcdn.apple.com/content/downloads/32/33/041-2303/W5jKtsQhhMgMXs3TKhPDqLXrpXyY2HdXMN/BootCampESD.pkg
http://swcdn.apple.com/content/downloads/33/54/041-2011/pRtCDYcWShMLxFggy3TzFzmfnnWQNFQBfJ/BootCampESD.pkg
http://swcdn.apple.com/content/downloads/53/34/041-4776/TpFRDjFyRJjSvtyj2vFsPyx2bH2NSYvwN9/BootCampESD.pkg
http://swcdn.apple.com/content/downloads/11/05/041-0925/g27es04pw9re5ggrfp3vuf8ew6r53asfz8/BootCampESD.pkg

Be patient as it’s probably 600MB.
Once your pkg is downloaded, double click it and install to a folder on your hard drive so you know where to find it.
The folder contains a nest of folders, the last of which contains a dmg disk image file. Double click to open. Voila. Here are your Windows installer files.
Copy them to a USB drive, or a burnable CD. It’s still 660MB or more, so it’s a full CD worth of burning time.
Done. You can now proceed with Boot Camp assistant Windows installation.



source: http://www.cafe-encounter.net/p824/bootcamp-drivers-direct-download-of-bootcampesd-pkg . The page was aimed at people struggling with BootCamp, but either way, it gets you to the BootCamp.pkg file, which contains the drivers. 

You still need open the OS X installer pkg file to get the the contents out, and to open a DMG file. If you are already in Windows, 7-zip will open it: http://www.cafe-encounter.net/p860/opening-a-bootcamp-driver-download-on-windows-7-or-8-with-7-zip