How can I switch my "left ctrl" and "left alt" key on my keyboard?

How can I switch my "left ctrl" and "left alt" key on my keyboard?

I'm working on a windows xp laptop at office. I have a standalone USB
windows keyboard that I want to switch the "left ctrl" and "left alt" only
on this keyboard.
I don't have any admin privileges on the laptop to run softwares to switch
the keys
I often pair-programming with others, and they don't want the keys switched
So I'm looking for a way to switch the keys only on my own USB keyboard,
is there any?
I know HHKB pro keyboard has some switches to switch keys, but that's too
expensive
Is there any adapter to connect the keyboard and the laptop, that we can
change the keys through it?
Can I modify the hardware of the keyboard to switch the keys, is it possible?