1. Check if you have enough disk space
Go into my computer right click on C: and go to properties, ideally should have more than half free.
2. Check computers specks
Right click on My Computer and take a look at how much RAM (bottom right hand corner) you have. This should be equal or over 1024mb any less you should be buying more RAM, this varies between 30 to 120 euro for 512mb to 2gb (2048mb). Vista will not run with under 2Gb or RAM.
3. Defrag your hard disk
* Open My Computer.
* Right-click the local disk volume that you want to defragment, and then click Properties.
* On the Tools tab, click Defragment Now.
* Click Defragment.
If you see lots of red then that is good thing means defrag will make a difference. This can take quite a lot of time.
4. Turn off things that you do not need running.
* Skype (really big killer for computers at startup) Right click on skype go into options and look for tick box that says start on startup.
* Google desktop search, I recommend just uninstall it.
Msconfig, go to start menu -> run type in msconfig and run, be careful what you turn off but can make all the difference.
5. Make sure you have all windows updates
Click on updates and run updates to windows, this should fix little and big security and faults in windows.
There are few more tips but generally these 5 should do 80% of the computers out there, could also be slow because of hardware failure or just computer getting old.
If you have any questions about this please do contact me for more insturctions or if one of steps does not make any sense.
n.b: If your computer is more than 3+ years then might just be getting old.