I presume you are using a Windows OS
Attempt to boot into safe mode.
Go to Run type msconfig go to startup
How many startup items are running.
Is there any with no text or you are not sure of.
remove the tick from some of them and reboot.
Other things to consider
You may have a virus.
Do a complete scan of your system.
Also scan for spyware.
I use spybot
Attempt to get your hands on a util called cclean.
Install and run it and clear out reported files and dead reg files.
Have you turned your router off of 15 seconds and turn it back on?
Are you using wireless?
Can you connect via etherrnet?