Why is my computer stuck?

I recently got back my Acer Laptop last Sunday after over a year of it being out of commission due to viruses. I have not gone to any disreputable sites (according to my anti-virus) and have only downloaded one thing: iTunes 10. I have also upgraded my Sound Forge Pro Software. For the past couple of days, it had been popping up with windows saying that random Windows programs were no longer working and it had also been acting very slow and every time it went into the Internet or another program, it would not respond. Now yesterday, it was working perfectly during the morning, yet during the afternoon, every time i would turn it on, it would take me to the start up menu thingy (safe mode, safe mode with networking, etc.) and whenever i click on something, it starts loading, shuts off and takes me back to this page. What could this mean? Please help!