Hi Pablo, you dont happen to have Kazza installed do you or has your brother been downloading from it
Its notorious for messing pcs up and I have come across this which may be your problem :
Infection Method
The dlder.exe spyware file, also functioning as a trojan dropper, is installed by Grokster (1.33), Bearshare (2.4.0b7), LimeWire (2.02), Net2Phone (unspecified versions) and KaZaA (unspecified versions). It may have also been installed by some versions of BonziBUDDY, but this has not been confirmed. The dlder.exe file is normally written to C:\Windows\dlder.exe. According to multiple sources, the user is asked whether or not they wish to install the "ClickTillUWin" component (carrier of the dlder.exe trojan), but the component may be installed even if the user chooses "NO".
Upon installation, the dlder.exe trojan first connects to the web site
www.2001-007.com and transmits data, including a GUID, the user's IP address and browser version. According to this site (Spanish), the request is in the form:
http://www.2001-007.com/index.asp?UserURL=GET+/&User_IP=127.0.0.1&userid=127&User_Browser=IE . This URL returns a numeric value that appears to count the number of unique installations.
The dlder.exe software then downloads and installs a trojan file named Explorer.exe from the same site, to C:\Windows\explorer\Explorer.exe (do not confuse this with the required Windows file explorer.exe, located at C:\Windows\explorer.exe). The dlder.exe file then places a Run key in the Registry so that the new Explorer.exe trojan runs at startup.
Manual Removal:
Terminate Dlder.exe and ExPlorer.exe using Windows' End Task (CTRL-ALT-DEL) dialogue, if possible.
Delete the files: dlder.exe (normally in C:\windows) and the phony Explorer file (normally C:\Windows\explorer\Explorer.exe). Be sure you are not deleting Windows Explorer, which is located at C:\Windows\Explorer.exe.
(Optional) Remove these programs' Registry Run keys under HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
Hope that this helps