|With 4GB RAM you should not be getting any RAM problems at all unless some of the RAM is no longer working (Check Control Panel >> System to see how much RAM Windows has detected).|
If your hard drive is getting full though, that will reduce the amount of "virtual memory" that's available to Windows and will generally slow everything down.
Another cause could be a bad malware infection which is eating up your computer's resources. Buy a good malware scanner (eg Norton Antivirus -- not a free one they all have weaknesses).
But even Norton can miss some types of malware. Get advice from the malware forum here: http://www.computing.net/forum/secu...