If your system cannot shutdown properly or became slow down a lot. You can try to fix it with the following suggestions.
1) Disable all the startup programs
First click "start" and "run" then type in "msconfig" in the input box and click OK button. Now you have start the system configuration utility program. Click the "Startup" folder which list all the programs that will run when the windows start. Click the disable all button on the right botton corner. Then click the OK button. Restart the system and you will found that no more startup program is running in the background. (After the machine get back to normal, then you can enable each of them to find out which one might cause the problems.)
2) Scan the system with Norton Antivirus or AVG
Put the Norton CD run install it into the computer. Then connect the internet to download all the latest update. Click the scan button to scan all the files. If found any virus, just fix it using the Norton options.
3) If your home page was hijacted.
Download the "Hijack this" in www.cnet.com. Backup the registry and check those items that might hijack your home page. If you don't know what the item is, please click the info button for explanation.
4) Install spyware program.
Download "Ad-aware" and "Spybot Search and Destroy" programs. Ad-awre have more options and Spybot can immune your system. If you want to install development tools, sometime you might need to undo the immune in order to make changes in the registry.
5) Install a software firewall
Even you have a hardware firewall or router, I highly recommended you to install a software one because it can block you from downloading virus and spywares. Or let you know who is scanning your ports.