After checking for infection using Rich's steps, if probs persist:
Can you get into the Event Viewer>Custom views>Admin to google repeat errors to see what is causing this? Type event in Start Search box.
Try SFC /SCANNOW Command - System File Checker
Do you have a Windows 7 DVD installer for your version of Windows 7? If so, place it in drive and run a Repair Install
If not, make your Acer E Recovery DVD's, copy out your files and run Acer E Recovery: Acer Support - Frequently Asked Questions list for How to use Acer eRecovery
There is a possibility you have an infection that will require wiping the HD to reinstall using the Recovery disks. If so and probs persist after recovery, wipe the HD using free Partition Wizard bootable CD to reinstall with Recovery disks or clean reinstall with a Windows 7 installer for your version using the Product Key on the COA sticker on bottom.