could be too many programs running in the background using up memory.or maybe even spyware.
some more info on windows version and set up would help.
to see how much ram is being used right click the taskbar and click task manager/performance
to disable programs running at start up with windows,go to start/run and type msconfig into the box and click ok
click on the start up tab and untick any programs you recognise.go to click here for referernce.but leave your firewall and antivirus ticked as you want these to start with windows.click apply and ok then restart.
you will now get a warning that you have changed the way windows starts,just tick dont show again.
then run disc cleanup,do a disk check and defrag.
try running spybot click here
remove disk doctor maybe
if its xp upgrade to 512 of ram