sorry... you can only run IE on Windows? I hope that was poorly phrased because I've seen IE on Linux and iOS.
As for Windows Update only being compatible with IE, that makes sense. Microsoft provides Windows Update, Microsoft provides IE; that's a circumstance where I don't see the developer needing to cater to other browsers. Besides, if you keep XP, Vista and 7 up-to-date, you get critical updates in XP without having to run IE for Windows Update. Vista and 7 have it built into the Control Panel now (at least what I remember of Vista), so you don't have to run IE at all. In fact, IE tells you to use your Control Panel when you attempt to visit Windows Update.
For those of you defending or making excuses not to upgrade IE7, the update is free. IE may not be the best browser out there, but they did put one thing in: compatibility mode. Microsoft thought far enough ahead to realize there are sites requiring older versions of IE still and tried to accommodate.
Considering security issues online, you should keep your software updated (be it IE, FF or Chrome). If you can't make the updated version work with a web page, I don't care if you're company or NPO needs to access a certain page, that page needs to be updated. It doesn't have to be fancy, it has to be functional and compatible. How many more updates will someone need to release before you (the developer of any IE7 compatible page) finally get that you must
update the page?
I agree with the comments earlier that relate to "Why are you shopping at work anyway". We're all mature enough (or should be) to know that we shouldn't be doing this unless it's business related.
As for the main post, I say don't bother with the "tax" and don't bother making the site compatible to any but the last two versions of a browser; in this case, IE 8 and 9. If it works with an older version than that, the user can consider themselves lucky and so can the IT staff at the company or NPO.