does it stall on start up or once windoze is running? once into windoze does it stall quicker each time you reboot?
Click START, then RUN and type MSCONFIG into the box and hit enter, you will then see "System Configuration Utillity" Click on STARTUP, you will see a whole load of stuff that loads on start up. Uncheck as many boxes as you can, read what everything is first before unticking. If you see anything related to printers, microsoft office, adobe, picture viewers etc then untick them. Leave anything to do with your chosen firewalls, antivirus etc. If you dont know what it is then type it into google to find out, disabling some things can stop your pc from responding. If you know how to do a screen shot then take one of your start up and post back. Your problem could also be related to missing or corrupt dll's, another very common problem. Have you installed or deleted any software recently?
The list for this type of problem is almost endless and could be caused by any one of a million things, think about anything you have done lately on your pc, new programs, antivirus,, cd/dvd burning software. These types of things dont like running with others and can cause easilly fixed problems.
As Brass Machine says, if you can then try ALT CRL DEL together to open task manager, click on processes then click on CPU twice, you will then see what is hoggin all your resources, you can then also click on mem usage to see what is stealing memory, dont be surprised to see your chosen antivirus program!!!
Need anymore help then just shout back, I have 3 mates that do this for a living and I'm sure between them and the guys on here your issue can be resolved
daz