there is nothing wrong with any of the progs you mention ... apart from AVG which is worth every penny!!
(try avira instead ... still free ... and far better detection rate)
all scanners will slow things a little ... (I assume you've noticed more of a hit just recently)
the basic rule is
1 virus scanner, 1 firewall, ... and as many spyware scanners as you can be bothered to use (but only one running in real time).
400Mb ram????
but 384 or 512 is OK (honestly you won't see much improvement from increasing to 1024)
your best bet is to have a good clear out ...
cc cleaner
http://www.google.co.uk/url?q=http://www.cclea ner.com/download&sa=X&oi=smap&resnum=1&ct=resu lt&cd=1&usg=AFQjCNGSbcQ6KcnsjlF5Y2QL4CkB3Ya2oQ
is good if you are not confident (just keep the defaults)
run scandisk (because you can!) and also run defrag
for all your drives (in my computer ... right click a drive letter then look in tools)
you should aim to keep about 25% of your disc free (that gives you a bit of leeway) and run CC about once a month
same for defrag ....