virus or what!!

av desktop compaq P4 machine-2.8Ghz,256Mb,80Gb-running on winxp Pro SP 1a. This machine started behaving funny with open windows shutting down and at times became painfully slow. I suspected a virus infection altho the machine had norton antivirus2005 installed-i figured out the antivirus was old so we bought norton 2006version. On trying to install this version, the machine refused and would automatically restart by itself midway-after severall attempts with the same result i figured out the machine was already seriously infected so i backed up all important data, formatted the hard disk fully and reinstalled windows afresh.... i then installed the new norton anti.v 2006 and as expected it loaded flawlessly, the machine was once more like new loading very fast and behaving like a machine worth its name-i slaved the hard disk where i had backed up the data and did a full scan using the norton2006-to my surprise! no virus was found-the backed up data turned out to be clean-since i culdnt believe that, i looked for other antiviruses i.e i installed and used McAffee 8.0---NO VIRUS DETECTED-AVG antivirus 7.0-NO VIRUS DETECTED-panda antivirus-NO VIRUS DETECTED-i convinced myself the infection was gone so i recopied the backed up data. After 2days the prob was back again the same way!!-i decided to look for the infection manually-i found that in my documents folder-most folders were replicated in that a folder named accounts had a similar folder inside it by the same name which when u open returns u to the my documents root in an explorer kinda window-several folders were like this apart from the my music, my webs and some of the other system folders. When i go to tools in the menu bar, the folder options tab in the tools is missing. When i check the properties of this replicated folders they funnily turn out to be apllications with .exe extensions. I searched the whole system and to my surprise found some folders with the same properties in the windows\system32 folder