Do you have a program called Microsoft Office Picture Manager? You'll find it in your start menu, under Microsoft Office->Microsoft Office Tools. If you have, run it and load in your image, and follow these steps:
1. Having loaded in your image, click on Edit Pictures on the top toolbar. You'll see a sidebar open up with different tools listed.
2. Click on Resize.
3. This gives you 4 options - Original size, Predefined width x height, Custom width x height and Percentage of original width x height. You need to select the 3rd option, custom width x height. Enter the value 100 into both
of the boxes underneath where it says Custom width x height. The program will calculate the new image size to allow it to fit into a 100x100 box - so if it started out at 200x150, it'll reduce it to 100x75, keeping the correct "aspect ratio". You'll see the new image size displayed at the bottom.
4. Click on OK, then save your image under a new name
- make sure you use the Save As option from the File menu, otherwise you'll lose your full-sized original.
This is easier than Paint, which (in my version, anyway!) only allows you to change by working on Percentage. If it all goes horribly wrong, you can just select Discard Changes from the Edit menu to get back to where you started.
You can also use the same program to "crop" an image, to pick out just a part of your image to use as your avatar (eg just your horse's head). Load the image as above, and click Edit Pictures, but then click Crop from the side bar. A set of "handles" will be put on your picture which you can drag to reduce the amount of visible image, with the part you've cropped shown faded. Having cropped it, you may then have to resize as well, to get it down to the 100 max.
Picture Manager comes as part of MS Office 2003 (with Word, Excel etc). If you don't have it, have a look to see if you have any other photo manipulation software - most digi cameras and phones come with something. They should all have some kind of resize option, which should work in a similar way.
Hope that helps - I'll try and add some screenshots as well
(edited to add screenshots)