you may have tried these things - but i'll give you step by step guide and follow these and this should speed up
you computer
+spyware sweepers for your computer - download from www.download.com - trusted site and will not have any false positive programs on there - they test them you know - when doing a scan turn off the automatic retore points
+ do a disk clean up - click on start then all programs > assessories > system tools > then disk cleanup
+ yes do a defrag BUT do it in safemode because not so many programs running in the back ground (when starting up press the f5 or f8 - select the safemode) again under safemode
+ check the programs that startup when windows start - these may be running in the background and will slow down your computer - click start then run - type msconfig - click on the tab for startup - this should produce a list with ticks in it - these are the programs that start when windows start (this process is totally reversable) you can disable what you wnat you can create a short cut on your desktop instead- try disabling them alll then click ok and restart - I bet this will speed up you computer - try to leave the you anti virus etc running - do you need any of these - if you don't know what any of these startup programs are or system files - just type the file into google
+ Get some more ram
+ do a check on your hard drive for bad sectors - goto my computer - right click on the hard drive - then properties - then tools - then check your hard drive for errors - make sure that both the check boxes are checked - you'll get a error then restart - this will take a while
+ do a system file check - click start > run > type sfc /scannow
+ check in the eventviewer for clues to see if anything is throwing out error codes > start > run > type eventvwr.msc - look under the application and system - look for any errors or warnings (double click on them)- again google the results
The next part only if you are comfortable with the computer
+ if you are comfortable with the registry - click start - run type - regedit - navigate to HKEY_CURRENT_USER/ControlPanel/desktop - modify the MenuShowDelay and reduce the value 400 - (this milliseconds) this will make the start menu display quicker
+ you can also speed up your pc also by cutting down on some of the special effects - right click on my computer > select properties > then click on the advanced tab then click settings under the performance tab > select the option for "adjust for best performance"
+ also try disabling the indexing services - this may slow down searches but if your not bothered about this this - this will speed up you system - click start then run type services.msc - scroll down the list until you find indexing services - double click on it - in the drop down box ensure the disabled is selected
also using the msconfig as before - check the services tab and then hide all microsoft services > check to see if there are any processes that you do not need - be alittle bit careful with this option - again google if you need to find the prog
+ if you are a advanced user you can improve you speed by enabling NTFS and write caching - but may have already set this up when you installed windows
+ and lastly but a bit extreme - do a full system recovery - i.e reinstall windows - you will lose any data but at least this will give you a computer as if it was brand new
Follow these steps and this will speed up your computer
Beware if using regedit be very very careful