No Execute

A friend of mine asked me for help in a XP pro pc. Any new programs would give an error message saying adress X could not be written and they would exit before being installed or when attempting to run them (firefox or openoffice). I found out after some hours that by disabling the No execute funcion on boot.ini would make it go away and run the programs ok. He has recently installed SP3 and only recently he tried to install additional software. He has an updated anti-virus I think. Anyway I analysed the logs of HijackThis which I was able to run before I disabled this (no execute) setting and there was nothing suspicious running in the system. So the solution is to let it off for now and I told him to run sfc /scannow to check if the is any system file altered. Is there any big risk of having this setting off? What else can I do to find the problem?

 

Google says it's probably a virus

 

DId a serch with malware bytes and got some trojans in a quarantined folder (from the antivirus I think) so nothing new. My guess is it's some definition that was chaged wich makes the os think new programs are trying to infect the pc (maybe it is enabling the no execute for mozilla and newly installed programs). But I can't find any other informations about DEP (besides a way o enable-it to all programs only system or disable it completely) wich is causing this problem. SO disabling it is the option. I found that some people had problemas with firefox crashing like this and had to disable it too. The difference is that their program termination message was clearly poiting the problem to DEP. In my case it is just an error saying memory address could not be written. So my only option for now is to disable it. Maybe I will try to run sfc /scannow and reenable to see if it fixes anything