Firefox 4 downloads iTunes just fine, in fact the first thing I do is download Firefox with IE and then never use it again. 🙂
It sounds like to me your XP machine is in severe need of professional "geek services" you might even want to consider a new machine with Windows 7.
It's really not a bad OS coming from Microsoft and all, you'll be quite pleased I think.
You just need to keep in mind to get:
1: A 64 bit dual core, pretty decent processor. Quad core preferred. Score 7,000 or more on this chart.
http://www.cpubenchmark.net/high_end_cpus.html
2: 4GB of RAM standard, expandable to 8GB or even 16GB or more if possbile.
3: For video games you need a decent descrete video card scoring 1000 or more (for a laptop, higher for a desktop) on this chart.
http://www.videocardbenchmark.net/high_end_gpus.html
4: Windows 7 Pro 64 bit (so you can use VirtualPC to run XP mode (free download from Microsoft)
5: 1 TB drive 7,200 RPM.