For the general Internet audience, you'd be better off telling the users that they could/should upgrade to a newer browser. After all, IE7 or less is run on less than 1% of the world's web (thankfully!), and you shouldn't be supporting such a minimal audience. For your specific audience, where money runs tight and upgrades are not forthcoming anytime soon, you'll have to choose a balance between usability/looks and browser support.
For this specific case, I'd recommend getting a version of jQuery 1.x, which supports IE6 and newer, and lets you write one set of code that will run across all versions of IE that are supported, Chrome, Firefox, Safari, Opera, etc. That would take care of your scripting. After this, you'll want to build a custom style sheet with lots of "important" modifiers in a conditional comment that will take care of the rendering issues in the older IE browsers. The actual amount of work required may be very little for some sites, but without seeing your page's complexity, it'd be hard to tell. You could potentially tweak this up in just a couple of days depending on your site's complexity.
However, if it's going to take longer than that, then I'd just tell the users that they may not have an optimal experience, and they should try out Google Chrome or Firefox. Chrome runs on XP SP 2 or newer, for example, is free, and is usually considered acceptable software by most IT departments, assuming disk space isn't an issue. In fact, Chrome can even install without administrative privileges (it installs in the user's local directory), assuming the user has any permissions at all.