@arbilab: this may have happened to me
Two nights ago my computer would barely boot up, kept freezing, and clearly something was wrong. I thought maybe a post-XP virus. I tried System Restore to pre-April 7 and it wouldn't work. In desperation, I tried a Restore day of mid-March and it worked, now the computer works good as new. I'm having trouble removing Security Essentials, but I have turned down the update/scan function to once a week (at 3 am when I will have the computer turned off). Avast! is running, does not seem to crash/burn just because I could not remove Security Essentials, and the machine is running as good as new. I need a few days off from work to do back up, then run the Win 7 upgrade and reinstall everything.
The system is a 3 Ghz dual-processor/3 GB RAM Dell system from 2009. I am not a gamer, I just use internet/mail/Office/streaming video. The video card lacks a HD output, but this is not a deal breaker since I have a 50" smart LED tv and can stream in HD there (the 24" monitor IS HD, has a DVI input, but there is no HD output on the computer, just VGA---I did not understand when I ordered in 2009 that I needed an HD output to fully enjoy an HD monitor---the monitor was offered as an upgrade option--mainly because it was bigger--- on Dell's build it yourself site, without warning me that an upgraded video card with HD output was needed to enjoy full HD).
The system works perfectly for my needs. Since I am going from 32-bit XP to 64-bit 7, I might consider upgrading RAM to 4 GB, which is the max it holds. Crucial.com will sell you a single 2 GB piece so you can remove the 1 GB module while keeping the other original 2 GB module. This is a $32 upgrade. I know the upgrade will mean a lot of work, but I can't see throwing out a perfectly good computer. If I needed it at a second location, then I might just upgrade it and buy a new Win 7 model for home with HD, but I have all the computers I need at work.
PS to keep Security Essentials from starting up, I used CC Cleaner's start-up edit list to remove SE from the list of programs that start when the computer boots up. This move is working: Avast! starts with boot up, SE stays off (no icon in the lower right corner tray. There is a built in Windows command to edit this list, but I prefer the CC version because it's sortable by column, you can alphabetize the program if looking for a program by name, or you can sort by yes/no status (yes = start at boot up) if you are trying to find a program you want to turn off (or on). My "yes" list is much smaller than my "no" list.