Sometimes the infections will be so severe that you will need to reboot into windows safe mode, you do that by clicking the F8 key right when windows starts up, then selecting the 'safe mode' option. This will allow the AV or Antispyware products to kill off things that were running as services in non safe mode.
If you can boot your computer normally, there is another way to boot into Safe Mode with Windows XP: Boot the computer normally. Close all open programs. Click Start, and then click Run; the Run dialog box will appear. Type msconfig in the box and then click OK. The System Configuration Utility should appear. Click on the BOOT.INI tab. Check the "/SAFEBOOT" option, and click OK. You will then see the prompt to restart the computer, click Restart. The computer will then restart in Safe Mode. When another box opens asking if you want to run in Safe Mode; click Yes.