Cako,
As far as I am aware, a virus cannot launch from a .txt file, and hence .txt files (as well as a number of other types) are excluded from scans in the interest of efficiency (not wasting time and effort). Note that you are able to save EICAR.txt, and open it without Avast grabbing it as a virus. but as soon as you use the extension ".com" it is immediately grabbed.
At the risk of making your day, if you named it EICAR.jpg as just one of many other extensions it will not register as a virus in the scan either.
The reason it is picked up in the individual scan (presumably you did this from the right click menu in Windows Explorer) is that particular scan defaults to a "scan all files setting"
Now if you really want to check all .txt, .jpg etc files, you can create your own custom system scan and include the "scan all files" setting, however that just adds a heap more (and arguably unnecessary) work.
On your other subject, the amount of resources used. The threat to PC's seems to grow almost daily, and so does the amount of work needed to try to locate and stop it. It is perhaps a case of how to handle that extra work load. Smarter methods can only do so much after which you have to throw more resources at it if you want to keep the time taken down (more memory, faster multicore CPU's), or if you stay with the same resources, then you must expect the task to load up the system more, and for it to also take longer.
In addition to the PC shown in my signature, I have Avast5 on a 1.6 Ghz Core Solo laptop with 1GB memory. That on face value should be a slower system than yours but I have to say I am pleased with the performance of the new version.
