One option is to use Wine. Wine is a translation layer between the Windows software and the Linux Operating System. This means that Wine will take the calls that the software makes to the OS and translate them into something Linux will understand.
Unfortunately, your success with Wine could be shaky. There are many applications that run extremely successfully under Wine (World of Warcraft seems to be one of them); others (iTunes 8) take some work to get them to work correctly, if at all.
You can check whether your program is supported by checking the Wine Application Database. The Application Database may be a little out of date, so you may want to do a quick Google search, also.
Although it is not a perfect solution, you do not need the actual Windows Operating System in order to use the software.
If your software is not supported by Wine, or if you would rather not mess with the configuration that is sometimes needed, a Virtual Machine may be the way to go.
A Virtual Machine is a platform that allows you to run a complete Guest Operating System on top of a Host Operating System. In this instance, it means running a complete version of Windows (guest) on top of Linux (host).
VMWare and VirtualBox are the two main vendors of virtual platforms. Both allow you to run multiple Operating Systems on a single PC and both have free downloads. I prefer VirtualBox, because it is a Sun Microsystems product that is completely Open Source, but other people may prefer VMWare because it has been around longer.
It doesn’t really matter which platform you choose. Either one allows you to install Windows and run the software that you need. It will even allow your guest machine to access the data and hardware controlled by the host machine.
Once you download and install the virtual platform, you can create a new guest Windows machine. The one downside of virtual machines is that you must have a copy of the Operating System you are creating. If you do not have a Windows CD, you will not be able to create a Windows machine.
Once the guest machine is set up, you can install and run any Windows software that you desire. If you absolutely must have Quickbooks or iTunes8, I strongly recommend this solution.
It is possible to continue to use your Windows software. The easiest way is to find an open source alternative that is designed for Linux. If an alternative will not work, you can run the actual Windows software through Wine or a virtual machine.