if your getting to the welcome screen you can get into safe mode. when you see the loading bar on teh windows screen keep hitting F8, it will give you the option
Yep, on computers I don't own, I just see the boot POST screen(which in your case would be when the big blue DELL letters pop up and it does a memory test) and start hitting f8 until it shows the windows boot menu options. Try running the AV you've already installed while in safe mode, or save an antivirus installer and a spyware remover on a USB memory and pop it in, since you won't have networking turned on while in safe mode.
If the AV/spyware tools fail to pick up anything, or if it still won't go past the Welcome screen, in Safe mode try to see if it will let you select a "restore point". Go to start/all programs/accessories/system tools and select a restore point from a date while the computer was still sort of working. Keep in mind you'll need to run the antivirus again, but hopefully you'll know which programs did the trick the first time, or how to install them without making your computer go south.
If you're trying to run an antivirus/spyware program in "normal" windows (out of safe mode) and it wont start or keeps crashing, the virus is probably the one causing this. You'll need to open the task manager (ctrl+alt+del and task manager or right click on the task bar and select task manager) and kill any suspicious programs that
your user has started; for example I recently killed some spyware that in the taskmanager tried to hide itself as iexplore.exe(internet explorer, while I had no browser windows open), after killing it I was able to run my AV tools.
As regarding to programs AVG Free is a good free antivirus, and spybot search and destroy, cccleaner and adaware are all free and combined should take care of any spyware/malware. If you're really tech savvy, also donwload Hijack this! and you can manually delete offensive programs from the start up lists. Beware that you
can make a serious mistake that will cripple your computer with Hijack this if you don't know what to look for.
Hope this helps