The most common method is through a virus or spyware. Many of the newer types disable the 'onboard" antivirus and spyware removal tools that you may have previously installed. To get thing started, please click on either or both of the links below and run the free scans they provide..
Housecall Online Scanner
Panda Online Scanner
Next, run the online spyware tools at the link below:
Next, if you can, please download, install, update, then run ALL of the free spyware removal tools from the links below. If you can't download them on your current machine, visit an friend or family member and download them there and burn them to a CD and then install them on the problem computer from the CD.
Sony Vaio Intel Celeron 1200 MHz 256kMB RAM, running on windows XP Home 2002 v5.1. System is not allowing any new programs to be installed - AND firefox no longer runs(only IE, not preferred). Adobe pdf files(running Adobe Reader v6.0) will not open in web browser or offline, and new programs (whether off CD or downloaded from web) will not execute. Thought it had to do with 100% CPU usage problem, but have cleared that through clean boot process and run into same problems not loading programs. When this is attempted, the CPU shoots straight back 100%, available memory shrinks down to 70MB +/-, and the system just hangs. Have to end process to recover. I've run the MS malware program - none found, also running trend pc-cillin, and have disabled real-time scan. ????