Could be any number of reasons. Those above are good things to check. Something is taking up resources leaving you little.
If it has suddenly occurred you may want to restore the system from an earlier date in the hope of removing whatever it is you've inadvertently started up.
Maybe it'll be faster if you boot into safe mode, where you can run scans for virus or other malware.
Or it could be you've too little memory maybe. Or too little hard disk space - have you saved something large recently ?