I'm a Windows user for 10 years, since Win95 till WinXP which i'm using right now. I recently reinstalled XP and since then i had this problem.
The thing is, after i open any folder and then switch to, let's say, Firefox for surf. If i press <SHIFT> it will take me back to that opened folder like if i pressed <ALT+TAB>. Very strange and annoying, i can't type capital letters and symbols on keys 1-9.
If i close that folder, pressing the same <SHIFT> will do nothing.

Any suggestions or solutions for this?

I use a simple Logitech Deluxe keyboard and WinXP SP2.

In Control Panel - Regional and Language Options - (tab) Languages - (select) Text Services and Input Languages

Ensure the keyboard you think you have is listed there. If not remove all.

Also in Device Manager uninstall the keyboard and reboot.

Advise results...........

Yeah fully agree, you should also consider install SP3.

It turns out this doesn't apply to every folder being opened. So far i noticed this with "My computer", "Shared Documents", "My Documents" and root folders of C: and D:
For example if "My Pictures" or "My Music" is opened, pressing <SHIFT> will not toggle it...

@elpmek
I didn't do anything in regional settings, i went to Text Services and Input Languages and there are 4 languages activated that i use (Serbian 2 kinds, English and Japanese). Should i remove all of them as you said?
I also went and reinstalled the keyboard from device manager, it didn't solve the problem.
Thank you for your time.

Very odd, don't bother to remove the keyboards. Do you have the same problem with every keyboard layout?

Try this:
Start - Run - (type) sfc /scannow (with the space)(Enter)

sfc = System File Checker restores files to their correct state.

This may require you to put in your XP CD.

I think it worked, after the restart the problem is no more, thank you for your support!