Computer problem can you help?

Over the past few weeks my computer has developed the annoying habit of stopping in mid-input. I can be typing in Word, inputting a cell in Excel or even sending out a question to Fluther and it will just stop.
The little icon will indicate it is thinking and then it will resume. If I have continued to type there will be letters or even words missing. For instance, if I were typing “annoying” and it stopped after the second “n”, I might end up with the word “anning”. It is really a pain with numbers. I have to double and triple check a string.
It could be some kind of virus but I have updated and run Microsoft Security, Malware and AVI and an old copy of Ad-aware to no avail.
The machine is an older dell updated to Windows 7. It has a 64 bit OS, 4 GB ram, an Intel Pentium Dual CPU E2180 processor and a single 250GB hard drive that is 53% full.
Any ideas or starting points to offer?