If you don't have a device manager or task manager, computer will not let you access certain files and programs, will not connect to the internet, runs very slow and doesn't respond....You more than likely have a malware issue going on here...That is why usasma provided the previous links...If you have tried to run these scans and have had no success then I would suggest that you open up a new topic in the "Am I Infected, What do I do?" forum in the security section and get further assistance from people that are experienced with malware removal.http://www.bleepingcomputer.com/forums/f/103/am-i-infected-what-do-i-do/
Following is a link for How to use the Event Viewer:http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
You may also try to restore your computer back to a time when things were working properly using system restore. It may or may not help with your problem. You can also try to restart computer, continuously press F8...Select start from the last good configuration...This also may or may not work.
If you have a valid XP CD you can also try and do a repair installation or even a clean installation, but if you choose the later, it would be best to back up any files or programs that you want to keep onto an external Hard Drive or at the very least burn them to CD's for re-installation after re-formatting and re-installation of XP.