Remove anything that is not needed at startup. Go to start - run - msconfig - top right hand tab startup. Take the tick out of everything other than firewall and virus checker etc. Things like office and various sound progs hog memory. After rebooting put tick in left hand box to not be told each time that you have altered the startup.
Then download and run CCLEANER (Free Version) to clean out all the "rubbish/temp files etc) which accumulate over a period of time. Safe to use - just click on Analyze then Run Cleaner on the default settings and you should notice a considerable difference.enter link description here