problem with starting in Safe Mode

I hope someone can help me - it seems my system has been infected with malware - I use avast antivirus which did not dedect it.

I was able to succesfully run a couple of malware removing sftware founf here however I cannot run combofix - I get the mesage that it is not a win 32 application.

I was able to run mgtools and malware bytes though.

Could anyone tell me wha I need to do next?

I have a windows XP service Pack 3 system

Thanks in advance