A non-corrupted install of Win8 on good hardware shouldn't behave like that, so I wouldn't start thinking about Win7 yet.
The lags make me wonder about hard disk issues: check your system event logs for disk errors. While you're in there look through the system and application logs for anything unusual, especially during the startup period.
If this is a self-built machine make sure you have the proper drivers for your motherboard and other hardware. Head over to the motherboard manufacturer's website for those.
Otherwise, if this is a recently built machine (or maybe even if it isn't) and you're comfortable with reinstalling the operating system, sometimes it's just easiest to start fresh. I know that's using an awfully big hammer to smash your problem, but sometimes that's the fastest way if the issue is non-obvious.
If your system came with repair disks or a repair partition, use those per your manufacturer's directions.
If you have a Win8 DVD, gather the needed drivers for your system (especially Ethernet so you can get on the net to download other software), boot from the Win 8 install disk, go into the repair command prompt and use bootrec to wipe the boot sector in case it is infected (see http://support.microsoft.com/kb/927392), then use the Win8 installer to wipe your disk partitions and reinstall the OS.
Of course, don't forget to back up your data (two copies minimum!) and make sure you have reinstall media for all of your programs before you start this process.
Windows 8 comes with all the needed drivers already. nothing to "gather" like win-7.