You say when you try to run task manager 'nothing happens' - this is obviously different to 'I get an error message'. One possible scenario is that the taskmgr.exe was infected and has been healed in a way that removed it's ability to actually run, otherwise you would gat an error message. It is not unusual for nasties to target taskmgr and to make a taskmgr.com which would run in it's place - in fact it's a known vundo thing
Probably replacing it with one from another PC or from a windows installation disk will fix it nicely although you may first need to run sfc /purgecache to clear the backup file. The sysinternals Process Explorer is much more granular though and I would recommend it, it even has the option to replace task manager built in. One great thing about it is that you can get at the individual exes that are being run by services.exe and restart them - something which you can not do with task manager - and then there is the ability to view every dll loaded by a process this too is great for spotting nasty stuff usually because it has no manufacturer name. Then there is the ability to load the image from memory rather than disk, it's just better all round.