Windows Live Mail has a very sneaky way of updating itself in a way that doesn't use Windows Update, it doesn't ask your permission, and it doesn't tell you it's done it. Back in April they came out with one of these that caused problems for a number of folks including me. When I uninstalled it and re-downloaded it from the MS web site, that resolved the problem, and it hasn't occurred again since. In my case that was back in the summer. The current version is Version 2011 (Build 15.4.3555.0308). You can find your version by clicking the down arrow in the upper left corner of the WLM window, then clicking ABOUT.
If the above is not your problem, here're some other things you can do to check your system.
You can check for corrupted system files. Open an administrator command prompt and run SFC if the above doesn't help. Click START, then type CMD in the search box, right-click CMD.EXE and click Run as administrator. Then from the command prompt type sfc /scannow.
Get your antivirus program up to date and boot into Safe Mode. Note that some viruses can hide themselves from your antivirus program in normal mode, so you really need to scan in Safe Mode. To get into Safe Mode when you first power up, hit F8 about once a second until you get the menu and select Safe Mode. Then run a full system scan.
Finally if all else fails, you can check the rather cryptic system event log. To do that click Start -> Control Panel -> Administrative Tools -> Event Viewer. Once in Event Viewer click on the system log and scroll through the entries looking for those flagged "error" to see if you can find hints as to where the problem could be.
When you get your system back in good working order, I urge you to back your system up to an external hard drive and make regular periodic updates to it.
I hope this helps. Good luck.