You've downloaded those things, obviously, because your system has problems. It's most likely that you actually have a virus which can identify the program files for antivirus and anti-trojan software, and vblock them from installing or running.

Either format your system and start over fresh, or post a topic in our Security section to see if someone can help you identify what is infecting your system and help you get rid of it. My advice would be the former. There's only so long you can keep patching over something that is rotten at heart, and your Windows installation, I'd suspect, isn't in very good shape!

yeah, I just reformated and immediately installed firefox and ad-aware so won't get it again.

You'll also need an up to date Antivirus program, preferably another Adware/Malware tool such as Spybot Search and Destroy, and if you have a broadband Internet connection you'll need a Firewall as well.

try installing the programs in safe mode (reboot computer and press f8 continuasly

i got the same problem... i only tyed with Avast thoug, but i got a lot off other programs that isnt working: ati catalyst center, bearshare, limewire. but battlefield 2 starts... until i press the play button, then it shuts down.... wierd. i think i will reinstall my pc, if i only had my XP cd X(. could anybody help us? Plz?

There's your problem. I have a feeling youre infected with something. Post in the Virus forum please.

If your anti-virus or anti-spyware program(s) wont run or open, you probably have a virus on your system that has disabled your anti-virus software.
To fix this you need non-standard, robust Anti-Virus software that scans your systems virtual memory before installation and before loading. I had a similar problem while running Norton and was told by Norton I would have to download a special fix tool. Instead I bought the Anti-Virus from www.discmaster.info and it installed and ran no problem! It found the virus straight away hiding n my virtual memory and I have never had any problems since. The anti-virus updates and scans automatically but uses less system recourses than Norton! and you get 1 years free updates.
To get yours just click on http://www.diskmaster.info/UK/software.htm or http://www.discmaster.info/USA/software.htm
Hope this helps!

Another possibility is that a competing adware program is already running. Only one should be running at a time.

You will if that's all you've got. As someone said, antivrus (AVG is free, or Avast), and firewall (well i guess you have windows turned on at least) will be needed. ad-aware only scans and firefox blocks the bare minumum