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i run the avast scan in safe mode found some trojan....and i cant move to chest so i just delete it
What is the malware/trogan name, infected file name, where was it found e.g. (malware name, C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast icon), Warning section, this contains information on all avast detections. This may help us with what the trojan does and possibly how to combat it or if other elements are at work.
....when i log in pc with normal mode....the avast start warn me that a trojan found.....they keep coming non stop.....and sometimes avast have block somethings from downloading ad- ware to my friend pc......how can i find that things that download ad- ware to my pc?
1. So you are now able to log on normally ?
2. This would tend to indicate that there are still undetected trojan downloaders on his system, the other two anti-spyware links I gave should also be tried, SuperAntiSpwware and SpywareTerminator.
3. An effective firewall should be capable of blocking unauthorised outbound Internet Connections. Does he have a firewall and if so what (XP's firewall doesn't provide outbound protection) ?
4. For adware I would suggest the AdAware and Spybot Search & Destroy links I gave.
now his pc is faster a bit than before....but still very slow.... 
So you/he might be a little further from the ultimate nuclear option. But, I fear if the other software and anti-rootkits mentioned don't resolve it you are rapidly approaching the previously mentioned option.
After so long their system is likely to be completely compromised with password stealers, backdoors, possibly rootkits hiding much more, etc. (you really have a task in front of you) that realistically they should be considering a format and start from scratch.
oops!! sorry i know why i cant run oot times scan now........i just enables it but still dint run it....sorry...i m just a beginner ......
And this is I believe the reason as previously mentioned (however, you have since mentioned being able to boot normally (?), so perhaps not):
If as you say they can't boot normally, that may be the problem although avast does its scan outside of windows the normal windows boot starts (you see the windows logo) and is then interrupted to start the avast boot-time scan. So the total lack of a normal windows boot may be the cause of this failure.
There are other anti-rootkits, you have only tried one of them, there are two others I gave links for and there is also the anti-rootkit, detection, removal & protection link I gave with even more. I only listed the ones which have been commonly recommended on these forums and by members with reasonable success.
I would believe that not only did your friend not have an AV, they probably didn't have a firewall (?) and if so that is probably long ago bypassed.
