Lets try a Selective Startup (clean boot) - this will help determine if a Service or Startup program is causing the issue
How-to run Windows in Selective Startup (Clean Boot)…
1. Click Start, click Control Panel, click System and Maintenance, click Administrative Tools, and then double-click System Configuration.
2. Click Continue, or provide Administrator credentials if prompted.
3. Click the General tab, and click Selective startup.
4. Under Selective startup uncheck Load startup items.
5. Click the Services tab, check the Hide all Microsoft services box, and then click Disable all.
6. Click the Apply, then OK, and then Restart.
If you still have issue, re-do the above steps and this time click on Normal Startup.
If this is a recent issue, have you considered doing a System Restore to a point before this issue started?
Running the System Restore utility…
1. Click Start, click All Programs, click Accessories, click System Tools, and then click System Restore.
2. Click Continue or provide Administrator credentials if necessary.
3. Click Choose a different restore point, and then click Next.
4. Select a Date and Time prior to first sign of the issue you’ve described.
5. If the date you need is in excess of (5) days click the Show restore points older than 5 days check box.
Important Notice: That System Restore affects Windows system files, programs, and registry settings. It can also make changes to scripts, batch files, and other types of executable files on your computer. Thus, any recently installed programs, updates, or changes to configuration will be lost if you use System Restore. However, make note that System Restore does not affect personal files, such as e-mail, documents, or photos, so it cannot help you restore a deleted file.