well buddy, in the past, when things like that have happened to me, it was something that i managed to let past my firewall that ate up bandwidth/ memory. my suggestion is this, you run your scan again, if it continues to take that long you have something in there that anti-malware and anti-virus cannot detect (has to be if they wanna keep their jobs). instead of running out and buying new hardware first, try reinstalling a fresh copy of your operating system from your disk (if it doesnt correct it u can always by new hardware later).
if you dont have a disk immediately copy your operating system from the partition its in to a disk, you never know when your hard drive will corrupt and you'll need a new one (saves u from having to buy a new operating system too). it takes me, cause i use to reinstall about once a year because of defragmentation that i couldn't get at (until i found a new program that defragments the operating system), about 2 hours to get from debugged hard drive (100% empty nothing on it) to up and running with security in place and work programs installed. updates take about another 2 hours since i'm running a 10 year old computer with xp pro on it (service packs are a b***h). usually that gave me back my speed, got rid of any problems the "company software" was unable to remove, while making it more affordable to run an already out of date computer. There were occaisions of bad hard drive sectors, "burned out" ram, and even a cpu failure (never had that happen again on any of my comps).