Odd IE / Firefox Problem

[fullbug]

Just recently bought a new desktop, Windows XP Pro, having and odd problem with both browsers....
Doesnt matter if Firefox is open or not, when I type any address into IE, it automaticly opens the page in a new tab in Firefox....
 I have made sure that IE is the default browser and Firefox is not, yet this keeps happening, having used both browsers for years this is the first time I have ever come across this problem, anyone have any advise?
I have no idea why IE would use Firefox to display pages like this.....

[fullbug]

Another thing, sometimes when IE starts I get this window....


[saving disk space - old attachment deleted by admin]

[fullbug]

Hmmm, from doing a google search this Null thing seems to be an IE 7 problem, thing is I'm still on 6....
This trouble only seemed to start today after I added a second user to the hard drive....
Maybe this could be moved to the software or hardware forum?

[Broni]

In Windows Explorer, go Tools>Folder Options>File Types
Make sure, that following file types are associated with IE, not Firefox:
htm
html
mht
mhtml
shtml
url
xht
xhtml
xml

[fullbug]

How do I get to Windows Explorer?
If you meant internet explorer, cant find that option in tools....
Never mind, found it....

[fullbug]

How can you tell? I cant really make sense of what I'm looking at....

[Broni]

Find the file extension that you want to make an association for...
html, for instance...
Is Internet Explorer listed as the default program?
If not...
Click Change
Browse to find the program you want to associate with html or select the program from a list provided.
For Internet Explorer browse to Program Files > Internet Explorer > iexplorer.exe

Follow same instructions for other file types, I listed in my previous post.