Welcome to the forum reitsma,
Really, the only fair comparision would be by using the same machine with the same specs ie: in a dual boot setup or two machines with identical specs.
My Core2Duo machine (see my sig for full specs) is a multboot with XP (32 bit), Win 7 Home Premium (32 bit) as well as Ubuntu Hardy and Intrepid (both 64 bit).
Both Windows installs are running the bare minimum of startup apps as well as having some items (low priority items) delayed start. With no delay start add 10 seconds.
Win 7 boots to a fully functional desktop in 40 to 45 seconds as opposed to XP's 90+ second boot time.
Have no way to compare to Vista as I've never installed Vista.
Boot times are measured with a stopwatch as well as the BootRacer app.
BootRacer - Test your Windows boot speed
Both OS's automatically login.
Ubuntu boots in about 60 seconds although the 64 bit installs may skew the results somewhat.
Since I shutdown my machine when I'm done using it (this is after all a notebook and should not be left running like many people do with Desktop machines) so boot times are important to me.
As to cold start of Firefox or IE8 add about 8 to 10 seconds if you start them right after Bootracer says you are booted.
The first start of Firefox or IE seems slower than subsequent starts.
I use an app called SandBoxie to run all my internet facing apps like my browsers and email clients.
I've modified my shortcuts to open these apps in a "sandbox" for greatly enhanced security.
This increases the cold start times by 10 or 15 seconds for my browsers but subsequent starts are noticeably quicker althoug slower than
running unsandboxed.
The greatly added security is worth the sacrifice in speed.
Once started "sandboxed" these apps perform just like they weren't sandboxed.
IMHO and experience, Win 7 is miles ahead of XP.
BTW, the look you complain about is customizable.